Skip central agent interval_task when keystone fails

There is a chance when ceilometer-agent-central try to trigger interval_task
but keystone is not available, then an exception is raised but not caught, the
outside oslo.loopingcall will stop calling the task function, finally leading
to the agent-central service is no longer available forever, even the keystone
is available again.

We can skip the particular interval_task when keystone is not available.

Change-Id: I0849cafadac8fb8a7670aaa4cc76dc708bdb25a1
Closes-Bug: #1287613
Merge "Skip central agent interval_task when keystone fails"