Abstracted out invoking 'mesos-fetcher'.

Manually rebasing and re-editing https://reviews.apache.org/r/21233/,
which is supposed to be replaced now by this patch.

Original description: "To test the mesos-fetcher (and the setting of
the environment) more cleanly I did some refactoring into a 'fetcher'
namespace."

Also moved fetcher environment tests to fetcher test file. Added two
fetcher tests.

Review: https://reviews.apache.org/r/27516