Added environment name to message sendt to slack.

I've added the Environment name in the message sent to slack to identify what machine that the step was called from.
In a clustered environment you typically deploy to more than one machine, and need to know to what machine that OD has deployed.

Example of messages sent:

OctopusDeploy [12:47 PM] 
Success
----------------
Deploy feeds release 1.0.200 to Dev on WEBDEV


OctopusDeploy [12:48 PM]
Success
----------------
Deploy feeds release 1.0.200 to Stage on WEB01PST


OctopusDeploy [12:48 PM]
Success
----------------
Deploy feeds release 1.0.200 to Stage on WEB02PST


OctopusDeploy [12:50 PM]
Success
----------------
Deploy feeds release 1.0.200 to Stage on WEB01PST


OctopusDeploy [12:50 PM]
Success
----------------
Deploy feeds release 1.0.200 to Stage on WEB02PST