Move all lints tests to consolidated 'linters' tox environment
Move all lints tests to 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:
- adds a 'linters' target which executes all required lint tests.
- removes previously seperated tox targets for lint tests.

Change-Id: Ic157a673aedb256353c04f44d2f70605132ba9ae

openstack/ansible-role-diskimage-builder