This adds the ability to decide, per image, if xenapi should use
the agent for servers created from that image.
This opens up the path to using config drive or the metadata
service with cloud-init to perform tasks like file injection
It uses the image properties that are copied into system metadata
to detect if "xenapi_agent_present"="true" on the image the server
was created from.
If the tag is not present, it defaults to the value
of the new conf setting "xenapi_agent_present_default".
Becuase the above setting defaults to False, it means that
the xenapi driver no longer waits for the agent by default.
DocImpact
fixes bug 1178223
part of blueprint xenapi-guest-agent-cloud-init-interop
Change-Id: Ie51a9f54e5b2e85fe4ebebb0aff975db296ba996
0357b01c12