Add upgrade helm functionality to plugin

[icstreispe]

feature request: Can you add a feature (goal) so I can use maven instead of using helm in order to build and deploy on cluster. So instead of these 2 instructions:

mvn clean install helm upgrade -i target/helm/repo/-.tgz --set env=uat

I would like to do something like this (or similar)

mvn clean site -Puat

or at least these:

mvn clean install mvn helm:upgrade -Puat