Plugins can know what instance they are executing on

Plugins can now know which instance they are being executed on via the self.environment dictionary available in event handlers. As an example, if a plugin is being executed on the canvas-test.canvasmedical.com instance, self.environment would look like:

{
    "CUSTOMER_IDENTIFIER": "canvas-test"
}

But if the same plugin was executed on the canvas-staging.canvasmedical.com instance, self.environment would look like:

{
    "CUSTOMER_IDENTIFIER": "canvas-staging"
}