Remove use of sslutils

sslutils is the only oslo-incubator module we use which registers any
config options, and we don't even use those config options at runtime.

The problem with us using oslo-incubator config options is that they
need to be exactly in sync with the oslo-incubator version of those
used by every project using oslo.messaging.

Avoid all this be inlining validate_ssl_version() until we have it
available in a real library.

Change-Id: Id3b0bb2e7ede33ede9b66025d1af113ae60cfc58
Closes-Bug: #1287542
Merge "Remove use of sslutils"