Add CentOS 7 to test kitchen for testing
Don't install linux-raid plugin on CentOS 7

C7 uses nagios-plugins 2.x which removes all of the contrib plugins including
the linux-raid. So this logic ensures that it doesn't install it. For now Fedora
still uses the older versions of nagios-plugions but we may need to fix this
later for Fedora too.
Use single quotes
Better name and cleanup array
Merge pull request #4 from osuosl-cookbooks/ramereth/centos-7

Add CentOS 7 support