Generic tasks and ad-hoc task support

[macropin]

(Further to discussion in #667)

Strider currently assumes that all tasks conform to the "build->test->deploy" paradigm. This does not fit our use case. We are only concerned about the "Deploy" step, and a generic task model would suite us better.

We are using strider as a continuous delivery tool: Strider receives a poke via the GitHub / BitBucket plugin (webhook) and from there it performs some magic to deploy the new code on a Docker/Fleet cluster. We've had to shoe-horn this into the current model, and the inclusion of 'Test' buttons in our projects are unnecessary.

From a dev-ops perspective being able to define ad-hoc repeatable tasks would be useful too. Some examples could include: Task to update our cluster, or scale up or down.