Feature request: Add support for mocking capabilities

[omerlh]

The built-in .Capabilities provides options about the tiller runtime, e.g. the Kubernetes server version, supported APIs, etc. It will be nice to be able to provide them as flags to helm template, e.g. helm template --kubernetes-version 1.15.0.

[thernstig]

Definitely a good use case. A lot of us have complex if/then statements in our Helm templating, including checking the .Capabilities to check what the Kubernetes cluster support. Depending on this, one might want to run various combinations of the values.yaml file and check the output so it matches expected behavior.