default commands & values from env vars

the-panter-chart 2.0.0 no longer defines a default command.

this pr here adds default commands.

while iterating over this i found that the most flexible approach is to construct an additional values file from env-vars. Here is how this works:

in gitlab-ci, you can define DEFAULT_VALUE_ prefixed variables:

DEFAULT_VALUE_application_command: "my command"

which will get converted to

application:
  command: my command

This also allows to alter values based on job definitions and reduces the need for extra --set args in the helm command.

E.g. i could replace REPLICAS with DEFAULT_VALUE_application_replicas, which now can get overridden much more easyly

notice that values.yml (and values-prod, etc.) take precedence over these values (hence the DEFAULT prefix)