Fix error status code for multinic

When passing bad body in a request, most APIs return BadRequest response.
However, multinic API doesn't do it.
This patch fixes the error status code and adds a unit test related to
this change.

This patch's idea most came from
following patch: https://review.openstack.org/#/c/107266/

Change-Id: If1ef2077a0e9823dfc04e4ef5a6dfb02daf1a8dc
Remove unnecessary error log in cell API

There are unnnecessary error logs in cell API. It tried to log
error for invalid input which is not necessary because we already
raises exception for the bad request.
This patch removes them in both v2/v3 API.

Change-Id: Idd239903aa830f5d8fe50336dfa63a43b617584f
Catch CannotResizeDisk exception when resize to zero disk

When old flavor's root_gb is not equal 0 and new flavor's root_gb is 0,
the resize() in nova.compute.api will raise CannotResizeDisk.

Move up the new_instance_type check before using.

Closes-Bug: #1340159

Change-Id: I4abf93530cf919af50a88d6049019fb745547257
Merge "Catch CannotResizeDisk exception when resize to zero disk"
Merge "Fix error status code for multinic"
Merge "Remove unnecessary error log in cell API"