Prototype ansible plugin

[sureshjoshi]

This is a non-exhaustive list of items to be reviewed before the ansible plugin is considered experimental and before I submit a PR to Pants. The scope of these is also representative of "an ansible plugin is pretty useless without X":

• [ansible] Support linting #4
• [ansible] Pull in local roles #47
• [ansible] Add support for ansible galaxy #48
• [ansible] Add inventory to target API #51
• [ansible] Add args to top-level configuration #52
• ...