Current cache size default for "ldap groups -> couchbase groups" map may not be sufficient in all use cases

Cache is in menelaus_users, and is called "ldap_groups_cache".
It seems like current default value (1000) maybe small for some users. We need to make it configurable, or replace it with an ets table (basically make it an index for the users storage), so it can accommodate all the ldap groups that we have links to.