Allow remote installation

[LCaparelli]

Today users who would like to install the Operator without OLM would use the steps described in our quick install guide. While it works great, the user must have the project's files locally, as it uses our Makefile which in turn runs the actual installation script (which also requires locally downloaded files).

kubectl is capable of fetching resources using HTTP, like so:

$ kubectl apply -f https://raw.githubusercontent.com/m88i/nexus-operator/main/deploy/operator.yaml

And yaml syntax allows having several yaml in the same file, meaning we could compile deploy/operator.yaml, deploy/role_binding.yaml, deploy/role.yaml and deploy/service_account.yaml into a single file and then simply apply it, thus rendering hack/install.sh obsolete.

Installing it remotely would be as simple as:

$ kubectl apply -f https://raw.githubusercontent.com/m88i/nexus-operator/<some path>/operator.yaml

The README quick install section would also need to be updated to reflect the new instructions.