Added template to enable or disable data centres in an F5 GTM

We host our websites on multiple machines across 2 data centres managed by an F5 GTM / LTM. When deploying websites we disable a data centre at the GTM (Global Traffic Manager) level, deploy code to those machines, re-enable that data centre, then do the same to a remaining data centre.

The submitted script makes it simple to enable or disable a data centre as part of the deployment process.
The script requires that a plugin tool (provided by F5) be installed. More information is available here https://devcentral.f5.com/articles/icontrol-cmdlets

We are currently using this Octopus step in production and it works fine.

Note : After a firmware update, we will be moving over to using F5's REST API to tell the GTM to enable / disable data centres (removing the reliance on this additional install), once we have moved to this i'll either update this script or write a new one.