Fix epel_enabled and RHEL support in bootstrap-os

Looks like `epel_enabled` was not configured for the epel install in `bootstrap-centos.yml`. Also, there were no conditionals that would trigger bootstrap for RHEL.

roles/bootstrap-os/defaults/main.yml

@@ -5,3 +5,6 @@ pip_python_coreos_modules:
 
 override_system_hostname: true
 coreos_auto_upgrade: true
+
+# Install epel repo on Centos/RHEL
+epel_enabled: false

roles/bootstrap-os/tasks/bootstrap-centos.yml

@@ -52,6 +52,7 @@
     name: epel-release
     state: present
   when:
+    - epel_enabled
     - not is_atomic
     - package_python_pip.results | length != 0
 

roles/bootstrap-os/tasks/main.yml

@@ -18,7 +18,7 @@
   when: '"Fedora" in os_release.stdout'
 
 - include_tasks: bootstrap-centos.yml
-  when: '"CentOS" in os_release.stdout'
+  when: '"CentOS" in os_release.stdout or "Red Hat Enterprise Linux" in os_release.stdout'
 
 - include_tasks: bootstrap-opensuse.yml
   when: '"OpenSUSE" in os_release.stdout'