Add CentOS to list of Jenkins CI test systems

[jodh-intel]

We need to ensure the runtime and virtcontainers (see clearcontainers/runtime#730) work seamlessly on CentOS, so tests all PRs (and master) on a CentOS node.

[grahamwhaley]

/cc @chavafg - something you can bring up I suspect?

[jodh-intel]

@sboeuf, @chavafg, @grahamwhaley - could we come up with a plan to get CentOS added?

[jodh-intel]

I guess this is effectively blocked on clearcontainers/runtime#463.

[jodh-intel]

Strike that last comment - we're not blocked on CentOS, so we should be able to progress this.

[jodh-intel]

Hi @GabyCT - could you take a look at this? Once we have the CentOS node(s) installed, I think it should be as simple as:

• Create https://github.com/clearcontainers/tests/blob/master/.ci/install_centos.sh
  (which will just call $GOPATH/src/github.com/clearcontainers/runtime/installation/centos-setup.sh).
• Add a call to install_centos.sh if the OS is CentOS after the call to install_runtime.sh in https://github.com/clearcontainers/tests/blob/master/.ci/setup.sh.
• Making the new jenkins CentOS config file https://github.com/clearcontainers/tests/blob/master/.ci/setup.sh.