This commit removes Paramiko and refactors gpssh-exkeys to work
without it.  Specifically, gpssh-exkeys now assumes that SSH keys
between the master and each segment host are already set up before it
is run, and now only exchanges keys among the segment hosts.

We also take the opportunity to remove some dead code and improve some
file handling.
Co-authored-by: NDavid Krieger <dkrieger@pivotal.io>
Co-authored-by: NJacob Champion <pchampion@pivotal.io>
Co-authored-by: NShoaib Lari <slari@pivotal.io>