Enable CentOS7 CI in Travis

[arm4b]

This implements #68

Enable CI for CentOS7 (kitchen-docker in TravisCI).

Allow CentOS7 build to fail in Travis until we fully implement platform support in Ansible playbooks as part of https://github.com/StackStorm/ansible-st2/milestone/2 plan.

[arm4b]

As a workaround for CentOS7 we use different approach comparing to CentOS6 #81

As @Mierdin discovered before non-root login is blocked on current CentOS7 image, so we use root as simplest approach.

Additionally, I created CentOS7 packagingtest needs re-build #38 with detailed description so we fix it in future.

[arm4b]

So CentOS7 checks are running, SSH works, Ansible is installed and playbooks is executed: https://travis-ci.org/StackStorm/ansible-st2/jobs/192437248

CI started to fail on all existing platforms (incl. master builds) due to external dependency failure in ANXS.postgresql, which is unrelated to this PR.

I think we can merge @humblearner's PR #78 first which removes external ANXS.postgresql dependency (just in time!).

[arm4b]

Rebased on top of master which removes external role ANXS.postgresql dependency and the build is green again.

[humblearner]

👍 +1