Apply dns/ping/http checks only if the release image can't be pulled

Right now all the connectivity checks are executed simultaneously, and it doesn't seem necessary especially in the positive scenario, ie when the release image can be pulled without any issue.

So, the connectivity related checks should be performed only when the release image is not accessible, to provide further infos to to the user.

The initial condition for allowing to continue (or not) the installation should be related then just to result of the primary check (right now, just the pull image) and not the secondary ones (http/dns/ping), that are just informative checks.

Note: this approach will also help to manage those cases where, currently, the release image can be pulled but the host doesn't answer to the ping