Clarify Ansible installation for distros in docs.

For CentOS, install from distro packaging by default.  Explain
how to install on DEB based systems from packaging when they
update to a newer version of Ansible.  Finally, clean up the
ordering of the dependencies for a pip based install.

TrivialFix

Change-Id: I9564a03c2e75fd30b5ec9c7d64f5defb709d7fde