Allow Ansible package name to be overridden with an 'ansible_package_name' config option, for CentOS/RedHat

[mikemead]

Problem:
When using the CentOS extras repo, not EPEL, the Ansible package name is always ansible. The current behaviour assumes that the package name includes the major and minor version number, when you're trying to pin at a particular version.

i.e. yum install ansible2.3-2.3.0.0 vs yum install ansible-2.3.0.0

Solution:
To allow an older version of Ansible to be installed, from the CentOS extras repo, whilst maintaining current behavior; A new config option has been added ansible_package_name, which when combined with ansible_version will address the problem above.