MAR-125: Switching to Ansible 2.3.x:

- Updated documentation to reference only Ansible 2.3.x, removing mentions of
Ansible 1.9.x, and removing comments related to delayed switch to Ansible 2.x.
- Updated usage instructions to use Ansible 2.3.x.
- Updated pip requirements and its input file to use up-to-date versions of
Ansible and Molecule.
- Updated test site Ansible configuration to use pipelining.
- Fixed missing brackets in test site tls.yml playbook for "with_items.