Fix invalid LDAP filter for user ID with comma

The Keystone server would respond with a 500 error when configured
to use the LDAP identity backend and a request is made to get a
token for a user that has an ID with a comma. The response is like:

 Authorization Failed: An unexpected error prevented the server from
 fulfilling your request. {'desc': 'Bad search filter'} (HTTP 500)

This is because the user DN wasn't properly escaped in the filter
for the query to get the groups that the user is a member of.

Closes-Bug: #1302106

Change-Id: Ib4886e66af0e979fcf23a84bcd51b07034547cb9
Merge "Fix invalid LDAP filter for user ID with comma" into milestone-proposed