Remove pip_install meta-dependency
Remove pip_install meta-dependency

The RabbitMQ role no longer does any python package installation,
so the pip_install role dependency is no longer required.

Change-Id: I55f3749e680ea9ff496be7322c776d516ee75c0f

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