Update NovaServers.boot_server_associate_and_dissociate_floating_ip
It conforms with "Fix nova scenarios for assiciating fips" [1]
http://artifacts.opnfv.org/functest/2B3GHZXZMEIC/functest-opnfv-functest-smoke-leguer-rally_sanity-run-16/rally_sanity/rally_sanity.html
[1] https://github.com/openstack/rally-openstack/commit/
ab03bfeafda0a29def4c792a174ec1af836b68be#diff-e40d163061529de4f93597cc2597505325ededfaa93855d59b2f3c717f6fa460
Change-Id: Iaed6d2f8527f62629fb7a761860f789fa1e8df72
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>