hooks: introduce before/after_get_caps hook points

This patches adds a hook point that allows to modify the capabilities
which Vdsm reports on its getVdsCapabilities API. This hook can become
useful if a hook writer would like to fake the existence of networks or
cpu flags.

Change-Id: I1d76730e84df491225023b27d3b12b879d310cdf
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27679
Reviewed-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>