fixes #3892 - process REMOTE_USER_GROUP_N and REMOTE_USER_GROUP_#, add user to external user groups.

The configuration for this feature consists of enabling
```
LookupUserGroupsIter REMOTE_USER_GROUP
```
for
```
<LocationMatch ^/users/(ext)?login$>
```
and defining the mapping of external group names for EXTERNAL auth source to Foreman groups.