pin version of ansible-lint we use for testing
pin version of ansible-lint we use for testing

Latest release of ansible-lint 2.4.0 has broken
role linter tasks by introducing a dependency on
openstack-ansible-plugins for all linting

While we may choose to adapt the linter tox env
in the future, for now we want to return to previous
linter behavior.

Change-Id: I4eea5b03321bb10ac5a367aa8c6c5ff460813682

openstack/openstack-ansible-rabbitmq_server,openstack/openstack-ansible-rabbitmq_server