Port rescue API to v3 Part 1

This changeset only copies the v2 files (implementation and test)
into the appropriate v3 directories unchanged. The copy as-is
will not be loaded by either the v2 or v3 extension loaders. The
second changeset will then make the changes required for it to
work as a v3 extension.

This is being tried in order to make reviewing of extension
porting easier as gerrit will display only what is actually
changed for v3 rather than entirely new files

Partially implements blueprint nova-v3-api

Change-Id: I8dc2b42b4ffb4618978856d5520485cd34e726d9
Port rescue API to v3 Part 2

This patch contains the changes required to adapt the rescue
extension and the corresponding unittest to the v3 framework

Partially implements blueprint nova-v3-api

Change-Id: Iaa6d0cf7708b0887512db952b1e26ccde75d24ef
Merge "Port rescue API to v3 Part 1"
Merge "Port rescue API to v3 Part 2"