python ovs: Fix SSL exceptions with pyOpenSSL v0.13

python ovs: Fix SSL exceptions with pyOpenSSL v0.13

Centos provides pyOpenSSL version pyOpenSSL-0.13.1-3.el7.x86_64.
There are 2 issues using this version, which this patch fixes

 - The test case "simple idl verify notify - SSL" is skipped.
   This is because "python -m OpenSSL.SSL" is used to detect the
   presence of pyOpenSSL package. pyOpenSSL v0.13 has C python
   modules because of which the above command returns 1.
   So this patch fixes this by using 'python -c "import OpenSSL.SSL"'.

 - The SSL.Context class does not have the function "set_session_cache_mode"
   defined.  Our usage here was only relevant for server-side connections,
   (pssl), which is not yet supported by python-ovs, so just remove the
   usage of this function.  The default cache mode (server) will just
   be ignored.

I have not tested with older versions (< 0.13) of pyOpenSSL.

Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Acked-by: Lance Richardson <lrichard@redhat.com>
Tested-by: Marcin Mirecki <mmirecki@redhat.com>
Signed-off-by: Russell Bryant <rbryant@redhat.com>