Fix and re-enable test_disassociate_not_associated_floating_ip

Fixes test_disassociate_not_associated_floating_ip so it sets the
ec2_client class variable needed by a base class correctly. There
has been some bit-rot since it was disabled.

Note that this test will not pass until the nova fix
https://review.openstack.org/#/c/124320/ has merged.

Change-Id: Ice39358f4e8123334f24056daede68bead59729b
Related-Bug: 1179816
Merge "Fix and re-enable test_disassociate_not_associated_floating_ip"
Merge branch 'master' of https://github.com/openstack/tempest