sp: add setMasterDomain to StoragePool

It turns out that getMasterDomain is used mostly as a setter for
the masterDomain attribute. This patch makes it explicit renaming
the method to setMasterDomain.

Change-Id: I99caffb6143d3a4fff16102c3b7c00616373e193
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21659
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Maor Lipchuk <mlipchuk@redhat.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>