Add optional post-provision, pre-deis-install hook

[acr92]

A lot happens in the provision phase. However, some users (e.g. myself) have a need for executing something between the situation where the cluster is provisioned, and when Deis is installed.

Examples:

• Setting extra configuration parameters in Deis
• Deploying extra units to fleet (for logs, monitoring etc).

I think this could be solved by adding a extra, optional hook, which was executed by rigger during the provision phase.

Either that, or split provision into two commands:

• provision = get cluster up and running
• install install deis based on configuration