sd.py: Remove sds from sdCache when connecting

When connecting to a storage server, pre-existing domains should be
removed, since we have no way of knowing what happened to these
domains in the meanwhile.

Change-Id: Ic0e0d0e970ce55acf92f7e39ec9cf2170e948274
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=950055
Signed-off-by: Allon Mureinik <amureini@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19995
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>