Updating NAT class to use gateway interface

Also, passing CLI args to NAT constructor

fixes #437

use rcmd instead of quietRun when shutting down remote nodes

adding deleteIntfs option to switches and corresponding CLI command

use ControlPersist ssh option to fix ControlPath shutdown

fixes #448

Also adds docstrings.
Fixes #422

(pending) Create test_switchdpidassignment.py

IVSSwitch: turn off verbose logging by default

Most users don't need this much logging and it slows down the switch.

use ControlPersist ssh option to create a ControlMaster connection that will not die when a node dies

fix sudoers file to allow -u option