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.