Remove Openshift specific functionality from Kubernetes Maven Plugin (KMP)

[manusa]

Description

As of now, both Maven plugins (Openshift + Kubernetes) are just ports of Fabric8 Maven Plugin (FMP) with different names.
FMP provides functionality for both cluster brands. AS KMP is just a port of FMP, we can still see traces of OpenShift specifics in the codebase (e.g. s2i, openshiftPullSecret, openshiftDeployTimeoutSeconds...)

All of these properties and configurations should be moved to the appropriate plugin.

Tasks:

• Modify affected Mojos
• Update plugin documentation to reflect new state + flags
• Change jkube-website to generate documentation for both plugins using the configurable kubernetes-maven-plugin-doc

Relates to:

• Rename Plugins' artifactIds to kubernetes-maven-plugins and openshift-maven-plugins #53