[ADMIN_API]: Added OFFLINE checker and bug fixes

There is now a periodic checker that will test the connectivity to
all HAProxy workers. If the per-minute connectivity test fails more
than the limit number of times (10 by default), the nova device will be
deleted and a new one allocated.

This submit also stops allowing API updates to loadbalancers, nodes,
monitors, and logs for devices that are in the ERROR state.

Change-Id: I64e0029a33f4ca02917b2993b7dd9937a92a428b