rpm/cephadm: move HOMEDIR to /var/lib and make scriptlets idempotent on SUSE

rpm/cephadm: move HOMEDIR to /var/lib and make scriptlets idempotent on SUSE

Also introduce a cephadm group. Since the cephadm package does not
require ceph-common, the ceph group is not available to be used.

Drop the -r option to useradd, because cephadm does not qualify as a system
user.

Finally, fix the SUSE RPM build. SUSE RPM builds using "osc" implement a check
that compares the cephadm package's "before" and "after" file lists, to enforce
idempotency of the scriptlets. The "userdel cephadm" in the %postun breaks this
check, so omit it from the SUSE RPM builds.

Fixes: https://tracker.ceph.com/issues/43285
Signed-off-by: Nathan Cutler <ncutler@suse.com>