Provide generic Habitat task

[russellseymour]

At the moment all of the tasks that Habitat can perform are being encapsulated into separate tasks, e.g.:

• Habitat Build
• Habitat Package Install
• Habitat Export

This means that other features of Habitat are not exposed to the pipeline

These will be replaced by a single Habitat task that will be configured using task parameters to perform the appropriate task. The other tasks such as installing Habitat, writing out configuration files, exposing build vars will all stay - this just affects tasks that run the hab command.