Add environment at play level.

(See https://groups.google.com/d/msg/ansible-project/w3aqRqZ1hIw/SCMV3qwaE-wJ). Play-level environment is used for every task in the play unless task's environment is set. Task environment overrides play environment (i.e. they are not merged).

Future thoughts:

* Merge play + task environment rather than override or make it optional?
* Add environment settings to roles.