Introduce new rhev_build spec file variable and use it for mom

This new rhev_build variable will contain 1 if VDSM is being
built in brew as part of the official RHEV product.

It will contain 0 otherwise (oVirt and Fedora).

This allows us to adapt to slightly different versions and/or
package names in RHEV environment.

Bug-Url: https://bugzilla.redhat.com/1143992
Change-Id: Idb970f96a076339cd853001b2d7c2396401f5d13
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35673
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>