Fix volume migration VolumeType exception

Fix volume migration VolumeType exception

Since Iabf9c3fab56ffef50695ce45745f193273822b39 modified the default
loaded attributes, the obj_attr_is_set check in finish_volume_migration
is now True for VolumeType. This should not get copied to the new
volume (since it's lazy-loaded based on the volume_type_id), so
this patch adds volume_type to the list of keys to ignore. To get
test coverage, test_finish_volume_migration is updated to use the
default expected_attributes and dict comprehension is updated so
it succeeds without glance_metadata being present.

Change-Id: Id3670dba79afb0d9d1b686ac2772c955372e97af
Closes-Bug: #1555862
Depends-On: I2caf4d3f4aa088d099548e6e88d1776b4cc5810c