Use mod_version to clean-up apache version matching

This change uses mod_version (shipped by default on everything we care
about) to set-up version-specific config within apache rather than
within devstack scripts.

Clean up the horizon and keystone config file generation to use the
internal apache matching.

Since I6478db385fda2fa1c75ced12d3e886b2e1152852 the apache matching in
'functions' is actually duplicated.  just leave get_apache_version in
lib/apache as it is used for config-file name matching in there.

Change-Id: I6478db385fda2fa1c75ced12d3e886b2e1152852
Merge "Use mod_version to clean-up apache version matching"
Merge branch 'master' of https://github.com/openstack-dev/devstack