Adding support for instances without an image.

In Grizzly instances booted from volume no longer have an image
attribute and in fact no longer require a bogus image parameter
when booting. This applies the fix from novaclient to our version
of the _print_server(...) method.

Fixes OS-224

Change-Id: Ifdb353f41fe4b4492ced1013c24522bd535d82fe
Merge "Adding support for instances without an image."