fixes https://fedorahosted.org/sssd/ticket/1628

When user's alias is same as it's name, don't use it for searching in
sysdb, and for deleting.

https://fedorahosted.org/sssd/ticket/1612

This patch changes the handling of ghost attributes when saving the
actual user entry. Instead of always linking all groups that contained
the ghost attribute with the new user entry, the original member
attributes are now saved in the group object and the user entry is only
linked with its direct parents.

As the member attribute is compared against the originalDN of the user,
if either the originalDN or the originalMember attributes are missing,
the user object is linked with all the groups as a fallback.

The original member attributes are only saved if the LDAP schema
supports nesting.

Using the convenience function instead of low-level ldb calls makes the
code more compact and more readable.

The domain can be read from the sysdb object. Removing the domain string
makes the API more self-contained.

https://fedorahosted.org/sssd/ticket/1506

Changes how the new autofs entry objects are handled. Instead of
creating the entry on the cn=autofs,cn=custom level, the entry is
created below the map it belongs to.