cleanup: move _updateTimestamp to API.py

After Engine issues a setupNetworks or editNetwork API calls, it follows
by repeated ping() calls. If networking has been configured correctly,
the ping() is processed by Vdsm, and _updateTimestamp notifies helper
tools that the network configuration should not be rolled back.

This patch extends this behavior to all users of the API, not only XMLRPC.

Engines of versions before 3.1 used to call getVdsCaps() instead of the
lightweight ping() (Engine commit 47c61adeb133d95b has changed that).
Even older Engines (before commit d87de762dbce2ba7) and modern Engines
in editNetwork flows, do nothing special and depend on their periodic
list() call. For backward compatibility, we add _updateTimestamp on
these specific verbs, too.

Change-Id: I4270df333ff5ff56e4375742cdc526ec76ed3bf1
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23050
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Moti Asayag <masayag@redhat.com>