Do not require EPEL repo on RHEL-based target hosts

This change makes kolla-ansible more compatible with
RHEL which does not provide epel-release package.

EPEL was required to install simplejson from rpm
which was an ansible requirement when used python
version was below 2.5 ([1]). This has been obsolete for
quite a time so it's a good idea to get rid of it.

This change includes update of docs to read more properly.

[1] https://docs.ansible.com/ansible/2.3/intro_installation.html



Change-Id: I825431d41fbceb824baff27130d64dabe4475d33
Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>