Add optional Ansible provisioning for centos 7

[GR360RY]

Adds optional Ansible provisioner for centos7 that allows to customize image with arbitrary ansible code.

Relates to #120

[GR360RY]

@jurekh and @alexsander-souza , please take a look at this pull request. The change is implemented for centos7 only, but if you like the idea, I can provide PRs for other distros as well.

I'd like to hear your thoughts on the idea of switching to ssh communicator as a default option even if nothing is provisioned with ssh/ansible.

[alexsander-souza]

Hi @GR360RY, awesome work.

unfortunately, we cannot accept this PR as it is. Although we certainly don't oppose using Ansible to customize the images, this is not something we are willing to endorse and support out-of-the-box.

I propose we split this patch in two parts, one generic and another Ansible-specific. The former should include only the SSH-communicator related changes, and this is something we are comfortable to merge and maintain in packer-maas. The latter, the Ansible provisioner bits, should become a discourse topic that we can refer to in the README.

[alexsander-souza]

please, include only the SSH-communicator support

[GR360RY]

No problem. I'll refactor the pull request