Cap cryptography version

(cherry picked from commit ebbd6d5)

Author: zhaoqin-github

requirements.test.txt

@@ -7,6 +7,7 @@ pytest==3.2.1
 pytest-cov>=2.2.1
 git+https://github.com/F5Networks/pytest-symbols.git
 python-coveralls==2.7.0
+cryptography<3.3
 pyOpenSSL==16.2.0
 requests-mock==1.2.0
 netaddr