As part of deprecating legacy drivers, we're moving the env package to a
new drivers/shared tree, as it is used by the modern docker and rkt
driver packages, and is useful for 3rd party plugins.
The default job here contains some exec task config (for setting
command and args) that aren't used for mock driver. Now, the alloc
runner seems stricter about validating fields and errors on unexpected
fields.
Updating configs in tests so we can have an explicit task config
whenever driver is set explicitly.
