Version detection for CentOS 7

* Detect when runing on CentOS 7
* This commit also makes all tests pass on CentOS
Merge pull request #1216 from gabriel-samfira/centos-version

Version detection for CentOS 7

This is the first PR in getting CentOS 7 support into juju-core 

* Detect when runing on CentOS 7
* This commit also makes all tests pass on CentOS



(Review request: http://reviews.vapour.ws/r/525/)