Convert all of the fedora28 container usage to centos8

This change moves all of the fedora28 container images to centos8 to ensure
we're maximizing coverage in all of our roles with OS images that represent
real deployments. While fedora28 was useful to exercise RHEL8 code path's
while there was no RHEL/CentOS8 container image, we can now use the upstream
CentOS8 image ensuring we're getting a true, representitive, environment.

> In the process of testsing it was noticed that the test_deps role was
  triggering scenario jobs. This role is only used for test purposes and
  should not test scenarios deployments.

Molecule requirements have been updated to ensure we're testing with package
versions that reflect the realities of production deployments.

Hacks that were added to enable fedora28 testing have been removed.

The ptp role now only will test the ntp stop scenario on centos7. This is
done because there is no NTP package/service in centos8.

Change-Id: I0e960f9ee230c82edb2ad9d40557aa6d0237dfc0
Signed-off-by: Kevin Carter <kecarter@redhat.com>

_skeleton_role_/molecule/default/Dockerfile.j2

@@ -22,7 +22,7 @@ FROM {{ item.image }}
 {% endif %}
 
 RUN if [ $(command -v apt-get) ]; then apt-get update && apt-get install -y python sudo bash ca-certificates && apt-get clean; \
-    elif [ $(command -v dnf) ]; then dnf makecache && dnf --assumeyes install python sudo python-devel python*-dnf bash {{ item.pkg_extras | default('') }} && dnf clean all; \
+    elif [ $(command -v dnf) ]; then dnf makecache && dnf --assumeyes install sudo python*-devel python*-dnf bash {{ item.pkg_extras | default('') }} && dnf clean all; \
     elif [ $(command -v yum) ]; then yum makecache fast && yum install -y python sudo yum-plugin-ovl python-setuptools bash {{ item.pkg_extras | default('') }} && sed -i 's/plugins=0/plugins=1/g' /etc/yum.conf && yum clean all; \
     elif [ $(command -v zypper) ]; then zypper refresh && zypper install -y python sudo bash python-xml {{ item.pkg_extras | default('') }} && zypper clean -a; \
     elif [ $(command -v apk) ]; then apk update && apk add --no-cache python sudo bash ca-certificates {{ item.pkg_extras | default('') }}; \

_skeleton_role_/molecule/default/molecule.yml

@@ -16,9 +16,9 @@ platforms:
       http_proxy: "{{ lookup('env', 'http_proxy') }}"
       https_proxy: "{{ lookup('env', 'https_proxy') }}"
 
-  - name: fedora28
-    hostname: fedora28
-    image: fedora:28
+  - name: centos8
+    hostname: centos8
+    image: centos:8
     dockerfile: Dockerfile
     pkg_extras: python*-setuptools
     environment: