[ci skip]

While not technically invalid, it is not necessary to have the 
`memberOf` OID in the `user_filter`. It clutters things up and causes
confusion for users so it's better if we remove it from the docs. 

A customer noted an error/lack of clarity in the LDAP documentation
with the `ldap_search` example. Previously, if taken literally, the
customer may have expected the `$` variables to be automatically
replaced or if they paste the exact `user_filter` contents the
parentheses would have been incorrect. Let's just simply the filter
and use exactly what's in the configuration.

It was previously possible for invalid credential errors to go unnoticed
in this task. Users would believe everything was configured correctly and
then sign in would fail with 'invalid credentials'. This adds a specific
bind check, plus catches errors connecting to the server. Also, specs :)

Signed-off-by: Roger Meier <r.meier@siemens.com>

[ci skip]

[ci skip]

Further documentation about limitations directly impacting settings of
users' LDAP servers.