Remove KeypairConstraint and FlavorConstraint to nova.py

All nova custom constraints should be defined in
nova.py as neutron custom constraints.
And this change also remove the corresponding tests
to test_nova_client.py

Change-Id: Ie40192b5d53d60d2d4e152801cbd8583e6c45dac
Implements: blueprint nova-custom-constraints
Merge "Remove KeypairConstraint and FlavorConstraint to nova.py"