Return serialized node to the ramdisk

Implements: blueprint returning-to-ramdisk
Change-Id: I5536b2697416569aa3f7a5b969cfc5ef539db602
Merge "Return serialized node to the ramdisk"
Extend node_cache.pop_node() result to be a structure

In the next patch we will need `started_at` to calculate timeout for
waiting for power off state. We will probably need other fields that
might be stored in the cache.

Change-Id: I9d7107322530c5ac6e407a374acb52c4effe4563
Implements: blueprint returning-to-ramdisk
added an eDeploy plugin

It can talk to an eDeploy ramdisk to discover hardware.

Change-Id: I11b4cf21ec81ce8efd3222597a5226fe260721e2