Add toggles for modules to symbols file

Issues:
Fixes #660

Problem:
There are cases when you want to run the full test suite, but omit certain
checks based on the lack of modules in your bigip install. There is currently
no way to ignore these tests without getting creative in your test file location
to the py.test command.

Analysis:
This patch adds the ability to specify modules to turn "on" or "off" in a symbols
file when running the funtional tests.

Tests:
none needed

Author: caphrim007

symbols.example.yaml

@@ -0,0 +1,9 @@
+---
+
+bigip_netloc: "localhost"
+bigip_port: 10445
+bigip_username: "admin"
+bigip_password: "admin"
+modules:
+    gtm: false
+    ltm: true

test/functional/tm/gtm/test_datacenter.py

@@ -17,8 +17,16 @@
 
 from f5.bigip.resource import MissingRequiredCreationParameter
 from f5.bigip.tm.gtm.datacenter import Datacenter
+from pytest import symbols
 from requests.exceptions import HTTPError
 
+pytestmark = pytest.mark.skipif(
+    symbols
+    and hasattr(symbols, 'modules')
+    and not symbols.modules['gtm'],
+    reason='The modules symbol for GTM is set to False.'
+)
+
 
 def delete_dc(mgmt_root, name, partition):
     try:
@@ -73,7 +81,7 @@ def test_create_optional_args(self, request, mgmt_root):
             contact="admin@root.local",
             description="A datacenter is fine too",
             location="Between the earth and the moon")
-        assert False == dc1.enabled
+        assert dc1.enabled is False
         assert "admin@root.local" == dc1.contact
         assert "A datacenter is fine too" == dc1.description
         assert "Between the earth and the moon" == dc1.location
@@ -93,15 +101,15 @@ def test_refresh(self, request, mgmt_root):
             name='dc1', partition='Common')
         d2 = mgmt_root.tm.gtm.datacenters.datacenter.load(
             name='dc1', partition='Common')
-        assert True == d1.enabled
-        assert True == d2.enabled
+        assert d1.enabled is True
+        assert d2.enabled is True
 
         d2.update(enabled=False)
-        assert False == d2.enabled
-        assert True == d1.enabled
+        assert d2.enabled is False
+        assert d1.enabled is True
 
         d1.refresh()
-        assert False == d1.enabled
+        assert d1.enabled is False
 
 
 class TestLoad(object):
@@ -115,27 +123,27 @@ def test_load(self, request, mgmt_root):
         setup_basic_test(request, mgmt_root, 'dc1', 'Common')
         dc1 = mgmt_root.tm.gtm.datacenters.datacenter.load(
             name='dc1', partition='Common')
-        assert True == dc1.enabled
+        assert dc1.enabled is True
         dc1.update(enabled=False)
         dc2 = mgmt_root.tm.gtm.datacenters.datacenter.load(
             name='dc1', partition='Common')
-        assert False == dc1.enabled
-        assert False == dc2.enabled
+        assert dc1.enabled is False
+        assert dc2.enabled is False
 
 
 class TestUpdate(object):
     def test_update(self, request, mgmt_root):
         dc1 = setup_basic_test(request, mgmt_root, 'dc1', 'Common')
-        assert True == dc1.enabled
-        assert False == dc1.disabled
+        assert dc1.enabled is True
+        assert dc1.disabled is False
         dc1.update(enabled=False)
-        assert False == dc1.enabled
-        assert True == dc1.disabled
+        assert dc1.enabled is False
+        assert dc1.disabled is True
 
     def test_update_samevalue(self, request, mgmt_root):
         dc1 = setup_basic_test(request, mgmt_root, 'dc1', 'Common')
         dc1.update(enabled=True)
-        assert False != dc1.enabled
+        assert dc1.enabled is True
 
 
 class TestDelete(object):

test/functional/tm/gtm/test_rule.py

@@ -17,8 +17,16 @@
 
 from f5.bigip.resource import MissingRequiredCreationParameter
 from f5.bigip.tm.gtm.rule import Rule
+from pytest import symbols
 from requests.exceptions import HTTPError
 
+pytestmark = pytest.mark.skipif(
+    symbols
+    and hasattr(symbols, 'modules')
+    and not symbols.modules['gtm'],
+    reason='The modules symbol for GTM is set to False.'
+)
+
 
 RULE = '''when LB_SELECTED {
    set wipHost [LB::server addr]