Fix tear_down a node with missing info

When tearing down a node, there could be some situations where the
instance info is missing some properties (image source deleted before
tearing down the node), so skipping node info validation. Instead,
perform a power validation to assure that the node can be power
managed.

Closes-Bug: #1335909
Closes-Bug: #1345334
Change-Id: Idb1ca720bbd19de1fccfe049e592136dbca2aacf
Merge "Fix tear_down a node with missing info"