remote-user: Fix .conf migration in %post

If the remote-user auth plug-in is installed but is not enabled at the
time of an upgrade, its httpd configuration file may not be present.  In
order to avoid causing an error during a package upgrade, only modify
the file if it is present.
Merge pull request #2941 from Miciah/fix-plugins-auth-remote-user-post-scriptlet

Merged by openshift-bot