Update hooks.rst for Ganeti 2.7
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 <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

mbakke/ganeti,yiannist/ganeti,ganeti/ganeti,ganeti/ganeti,mbakke/ganeti,andir/ganeti,grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,dimara/ganeti,apyrgio/snf-ganeti,andir/ganeti,apyrgio/snf-ganeti,dimara/ganeti,onponomarev/ganeti,yiannist/ganeti,apyrgio/ganeti,mbakke/ganeti,onponomarev/ganeti,bitemyapp/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,yiannist/ganeti,leshchevds/ganeti,ganeti/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,andir/ganeti