Uses the switch CLI via ssh. Stole heavily from snmpit_netscout (for minimal
set of functions actually used) and snmpit_dellrest (for guidance on trunk
operations and general clearifications on what functions should do).

Passes basic unit tests: enable/disable ports, create/remove VLANs,
trunk/untrunk ports.

Requires some additional configuration of switches:
 * add 'snmpit' user with password from DB
 * turn off the !?*#! pager via:
   conf t
   line vty 0-15
   length 0
   width 0
   exit
   exit