Use Tempest-lib's compute migrations_client

Compute migrations_client has been migrated to
tempest-lib- I0dbae04c479b9d742a3e51d1d60c372093c2c027

Now Tempest-lib provides this client as stable interface so Tempest
should start using this from lib and remove its own copy.
Also remove its unit tests.

This commits makes Tempest to use compute migrations_client
from Tempest-lib.

Change-Id: Ic969a9b10ce0157e1213f26960ba8bc70d974413
4 files changed