Make get_image_metadata() use Instance.get_flavor()

This cleans up the last use of extract_flavor() in compute/, aside from
resource tracker.

Related to blueprint flavor-from-sysmeta-to-blob

Change-Id: I2a5c2b5d2220a05ea7e921f7d7077c6880fbc5bb
Merge "Make get_image_metadata() use Instance.get_flavor()"