Moved grabbing/overriding the minionswarm user to parse-time

This change provides a single source of truth for what user the swam
will be running as.
Pass the minionswarm user through to salt-key

Otherwise salt-key would fail with a user warning and not write the key
which would then cause the minionswarm to fail when referencing the key.
Merge pull request #10238 from whiteinge/minion-swarm-custom-user

Fix minionswarm.py when running as non-root