Support configuring libvirt watchdog from flavors

The previous watchdog-related patch only enabled the watchdog
configuration to be set from the image metadata.  Now, the watchdog
can be configured from both the flavor extra_specs as well as the
image metadata, with the latter overriding the former.

Implements: blueprint libvirt-watchdog
DocImpact

Change-Id: I626eb8749c1b9a531e49f2471734d903694c5ee8
Merge "Support configuring libvirt watchdog from flavors"