quickstarters/pushToRemoteStage should support existing local git repo

[clemensutschig]

https://github.com/opendevstack/ods-jenkins-shared-library/blob/master/src/org/ods/quickstarter/PushToRemoteStage.groovy

assumes that the remote repo (created by the provision app) is always leading, as our quickstarters only supply template files that are copied locally into it, and committed (as apposed to being a fork with an existing commit history)

For our new concept of "ODS development quickstarters" we need the stage to support a filled / forked local git repo. So instead of always cloning remote as first step, the stage should check initially if the targetDir is a git repo or not.

In case it is (and that's new) - it should just do a commit and then push to origin.