Check for cert-manager CRDs instead of Service during installation

[jordojordo]

During the installation of the kubewarden-controller there is a step for installing cert-manager, in order for this step to advance it is checking for the existence of a Service with the label: app: cert-manager. This should be changed to instead check for the existence of the cert-manager CRDs rather than a Service and label since users could have installed cert-manager previously with their own specifications.

[flavio]

I think we can close this issue, since we are going to entirely remove the cert-manager dependency (see #489)