Suppressed misleading log in unshelve, resize api

If instance is booted from volume, unshelve and
resize instance unnecessarily logs 'image not found'
error and warning messages in the compute.log when
image_id_or_uri is passed as None to
nova/compute/utils->get_image_metadata method.

To avoid this error and warning message, calling
image_api.get() method only if image_id_or_uri is
not 'None'.

Closes-Bug: #1370019
Change-Id: Ibf8c02fa153fc096e4e8a1c054ef62c935006091
Merge "Suppressed misleading log in unshelve, resize api"