update to latest base images, generate cert... (#242)

* make build.sh a bit more generic so it's more closely aligned to plugin reg script and downstream version

Change-Id: Ieb49c304ef2735c19483c089b6c768313541b288
Signed-off-by: nickboldt <nboldt@redhat.com>

* make README a bit more generic so it's more closely aligned to plugin reg script and downstream version

Change-Id: Id12076bd563675ec1093329db61cda8463965bf1
Signed-off-by: nickboldt <nboldt@redhat.com>

* rename build/dockerfiles/fedora.repo to build/dockerfiles/content_sets_fedora30.repo for consistency/accuracy

Change-Id: I97fc9d44d471f24d45ff655a700a4f1a8b43be8c
Signed-off-by: nickboldt <nboldt@redhat.com>

* rename build/dockerfiles/content_sets_centos8_appstream.repo to build/dockerfiles/content_sets_centos8.repo and include baseos repo too

Change-Id: I593d5c0ba825409d95919a24b09e51630dbd9565
Signed-off-by: nickboldt <nboldt@redhat.com>

* clean up build/dockerfiles/rhel.install.sh - no need to merge commands

Change...