Pick from 'none', 'keycloakify' or 'mvn', default to 'mvn'. 'none' will
not create a jar, 'keycloakify' will create a jar file using only tools
available to native nodejs, no additional system library required.
Choosing 'mvn' will behave as before, starting maven in a subprocess.
The bundler can be chosen in `package.json` or via `KEYCLOAKIFY_BUNDLER`
env var.
This commit also adds `KEYCLOAKIFY_GROUP_ID` and
`KEYCLOAKIFY_ARTIFACT_ID` env vars, which will be used to
define group id and artifact id in pom.xml and pom.properties, if given.
