Allow "yum install awscli" for CentOS 7 or more recent versions

[nkadel-skyhook]

EPEL for CentOS 7 has recently and successfully activated an "awscli" RPM. I've tested it, it's workable, and it's more stable to work from an RPM for these operating systems than to run "pip install" where the chain of dependencies may replace, or interfere with, other RPM installations. It also seems to work quite well with Fedora. It's not available right now for CentOS 6., unfortunately.

I'd like to suggest an option to use "yum_package awscli" rather than "python_package", precisely to avoid this confusion. Would that be a welcome pull request?

[cmlicata]

@nkadel-skyhook, that seems reasonable. Make sure you add the kitchen tests as well as well as any server spec tests that are required and I would be happy to pull it in.