pool: ignore refreshStoragePool calls on the SPM

The refreshStoragePool command is an HSM command and should not be
issued (and executed) on the SPM. At the moment we just ignore it
for legacy reasons but in the future vdsm could raise an exception.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=882958
Change-Id: I2c275bfb15e4948c2e28ea21fac787c36bb48e97
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10450
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>
Tested-by: Daniel Paikov <paikov@gmail.com>