New Travis CI job for scheduler in the core repo.

[Dominic Kim <st...@gmail.com>]

Dear whiskers.

We(Naver) are working on the contribution of a scheduler and about to set
up a dedicated CI pipeline for the scheduler [1].
This will just define one more environment variable to selectively run
scheduler tests along with a dedicated Travis job. This is for some
downstream that conforms to the upstream build environment.
So that we can prevent scheduler tests from running on downstream repo by
default but at the same time we can guarantee the sanity of scheduler
components while contributing.

Once the contribution is over, all scheduler tests will be under the
existing unit/system tests pipeline just like the others.


-dom

[1] https://github.com/apache/openwhisk/pull/5068