Feature request: make cert-manager optional

[khos2ow]

Expected Behaviour

I want to use ofc-bootstrap against an existing and configured cluster, which already has cert-manager installed and configured on it. But ofc-bootstrap is always going to install cert-manager. I'd like to be able to:

1. make the installation of cert-manager optional
2. make installation of ClusterIssuer (templates/k8s/tls/issuer-*.yml) optional

Current Behaviour

None of the above.

[alexellis]

We're currently migrating to a new way to installation method and I would expect we could accommodate this request at the same time.

[alexellis]

/set title: Feature request: make cert-manager optional

[khos2ow]

Awesome, I can lend a hand for this if need be.