fence_gce: Adds existing operation checks and multiple plug support (#400)

• Update fence_gce.py

Makes the following adjustments to the file:

• Makes tab/spacing consistent on all lines
• Adds functionality to allow for multiple --plug and --zone options delimited by semicolon
  • This makes pacemaker fencing resources easier to setup (one will now suffice instead of many)
• Stops requiring --plug for generic commands that don't need it (for example: monitor)
• Adds checks for any pending operations in order to prevent multiple reset commands from being initiated. If a pending operation exists already, the agent will wait for that to complete instead of generating a new request.

• fence_gce: Updates multi-plug support

Changes split on semi-colons to commas so that it works with the native multi-plug functionality.

• fence_gce: make zones work with multi-plug support

This change makes sure that we pull zone out correctly if the user puts in multiple zones and multiple plugs.