Shift public ping checker into deployment stage

Shift public ping checker into deployment stage

Avoid public vip failover at the end of post-deployment
by moving ping_checker resource into deployment stage.
Also pingd constraint has been changed to be based on score.
If all the nodes failed to ping the default gw, the resource
ain't stopped and remains on the last node where it was.
In addition public_vip_ping task has been splitted out
to avoid race condition when ping_checker resource might
be started up on controllers before a primary one.

JIRA: FUEL-212
JIRA: FUEL-217
Change-Id: Id60ece479fce1ceabc47034f39997ddbaacd62d9
Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>