zpm: abstract _prepare_auth function
zpm: abstract _prepare_auth function

This function encapsulates the preparation of auth args, removing some
of the "clutter" from the `deploy_project` function.

This makes it easier to (potentially) compose small functions together
for multiple uses, and it makes unit testing easier.

zerovm/zerovm-cli,zerovm/zpm,zerovm/zpm,zerovm/zpm,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zerovm-cli,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli