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
Make compute/api.py::resize() use Instance.get_flavor()

This is actually a teensy change, but along with it come a lot of fixes
to tests that were still passing instance dicts to this method.

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

Change-Id: I6eb905730e4a61628a2ac94f752fc56786e952d2
Merge "Make get_image_metadata() use Instance.get_flavor()"
Merge "Make compute/api.py::resize() use Instance.get_flavor()"