ARI: Don't complain about missing ARI users when we aren't enabled

Currently, if ARI is not enabled it will still complain that there are no
configured users.  This patch checks to see if ARI is enabled before logging and
error or iterating the container to validate the users.

Review: https://reviewboard.asterisk.org/r/3391/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@411173 f38db490-d61c-443f-a65b-d21fe96a405b