[SSSD] SYSDB: User and group renames

[Jakub Hrozek]

The attached patch fixes a regression in the local domain tools where
sss_groupadd no longer detected a GID duplicate. The check for EEXIST is
moved one level up into more high level function.

The patch also adds the same rename support for users. I found it odd
that we allowed a rename of groups but not users. There is a catch when
storing a user -- his cached password would be gone. I think that
renaming a user is such a rare operation that it's not severe, plus
there is a warning in the logs.

Unit test was amended, too.