vdsm hooks: this patch provides after/before_network_setup hooks.

This enables the capability to interact with physical network
management interfaces, setting up extra parameters, or
interacting with switches, etc.

It also introduces a standard testing decorator for hook
testcases.

Change-Id: Iac5c6f57b300b3b1a2a9bfad4a7919f4d8a74707
Signed-off-by: Miguel Angel Ajo <miguelangel@ajo.es>
Reviewed-on: http://gerrit.ovirt.org/20076
Reviewed-by: Dan Kenigsberg <danken@redhat.com>