Allow any attributes in mapping

When using attribute mapping (e.g., user_additional_attribute_mapping), any
attributes that Keystone didn't know about were ignored and a warning was
issued, like

 WARNING keystone.common.ldap.core [-] Invalid additional attribute mapping:
 "description:description". Value "description" must use one of password,
 enabled, default_project_id, name, email.

This check is removed. Additional attribute mappings for "unexpected"
attributes (such as description) should be allowed. For one thing, all the
known attributes already have a way of setting the mapping, via the
user_*_attribute config option. This will allow setting a "description" or
any other attribute mapping for users.

Change-Id: Idc2cd75ecd3e1a06a50fd0a676d83c5de74ca7a5
Partial-Bug: #1293698
Merge "Allow any attributes in mapping"