Convert kombu SSL version string into integer

When specifying 'kombu_ssl_version' for the RPC driver such as either
"kombu_ssl_version=3" or "kombu_ssl_version=SSLv3" the relevant
OpenStack service (nova, cinder, etc) will fail as the underlying
rpc driver is trying to create an SSL socket which requires an
integer such as the following built-in SSL integer constants. Added
a validation step that ensures one can set only the supported ssl
versions and to convert from the specified string to an integer

Fixes LP# 1195431

Change-Id: I5d188f46a15bc4ba60d573d6b98def60c56cb987
Merge "Convert kombu SSL version string into integer"