Summary:
The schedule type is used to differentiate between a build run during
code review (and thus runs code that isn't yet in the master branch),
from continuous or other types of run that operate on landed code.

This doesn't change any behavior yet; this diff just adds
the plumbing to pass down an optional arbitrary schedule type string
from the CI system.

In the future, we'll use the schedule type to influence the behavior
of running tests.

Reviewed By: strager

Differential Revision: D15300120

fbshipit-source-id: 3b46afef2ff171b3fa095763dd5006a54ea328b8