Host Power-Management Refactor (#977674)

This patch refactors Power-Management to work with 'n' fence agents.

Fence agents now have their own table in the database. Each fence agent has an 'order'
field, and agents are run sequentially according to their order until the fence
operation  succeeds. When 2 or more agents have the same order they are run concurrently.

Power-Management health check was also modified to consider n agents.

API was enhanced to include 'n' or more agents. Now fence-agents have their
own API context: .../api/hosts/xxx/fenceagents. Agents can by added, updated and
deleted.

Change-Id: Id0b384347a52db8b0aa6ae684fad40a5491dd2f7
Signed-off-by: Ori Liel <oliel@redhat.com>
Signed-off-by: emesika <emesika@redhat.com>