Update hooks.rst for Ganeti 2.7

- The migrate instance hook now also executes on the primary node in
  the "pre" phase.
- Instance hooks now get full network information on each nic, if a
  nic's network is set.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>