Monitor bridges using vdsStats

Changes in vdsm/sampling.py are neccessary because previously no new
interfaces would be added to vdsStats. It would happen only after
restart vdsmd and therefore reinitializaton of HostStatsThread. Now
there is update of ifids and ifrates everytime sample is taken.

To give HostStatsThread time to collect data for statistics we skips
hotplugged interfaces what does not have all samples. In current setting
with AVERAGING_WINDOW==5 and SAMPLE_INTERVAL_SEC==2 it takes 10 seconds
to start giving information about hotplugged interface.

Change-Id: If73565bf62f339e338d65b0200a1ba7f1131b706
Bug-Url: https://bugzilla.redhat.com/675560
Bug-Url: https://bugzilla.redhat.com/988469
Signed-off-by: Petr Sebek <psebek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16227
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>