add non-routed subnet metadata support

implements blueprint metadata-non-routed

This patchset completes Quantum metadata support by adding metadata
proxy support for isolated network segments.  The support requires that
the guest instance request host routes, so that the DHCP port can be
used to proxy metadata requests. NOTE: The cirros image does not support
host router, so the UEC or equivalent required for testing and usage.

Change-Id: I962deef7c164ecb2a93b7af326ef8dca6e2b183a
Merge "add non-routed subnet metadata support"