add 'serial' varname to .panrc.  allows you to have tags which reference
a panorama managed device via redirection.  suggested by Jonathan Kaplan.

for example, given:

  hostname%0001C100522=10.0.0.1
  serial%0001C100522=0001C100522
  api_username%0001C100522=admin
  api_password%0001C100522=admin

$ panxapi.py -t 0001C100522 -Xjo 'show system info'|grep serial
op: success
        "serial": "0001C100522",

performs the API request on panorama managed device 0001C100522.