RFE: tag-source/git step implementer needs ssh key parameter

[itewk]

purpose

currently no way to use an ssh key to do git tagging (unless project happened to be checked out with an ssh key with write permissions).

suggested fix

1. add a utility function to import arbitrary ssh keys (like we have for pgp keys)
2. update tag-source/git to take an ssh key paramter and import it using the ultity function
3. any other updates to tag-source/git to use the ssh key