Revise all lint tests and add consolidated 'linters' tox environment
Revise all lint tests and add consolidated 'linters' tox environment

OpenStack-Infra has requested that lint tests be pipelined into the 'linters'
tox target in order to make more efficient use of nodepool resources.

This patch:

- Ensures that a complete set of lints tests is available in individual
  tox targets.
- Adds a 'linters' target which executes all the other lints tests.

Change-Id: Ie6f8f38e1d21863b2cfbb3cd2075a4f4bf8375e1

openstack/openstack-ansible-memcached_server,openstack/openstack-ansible-memcached_server