Use setConcreteOnly() in Phabricator and only list/launch concrete Daemons

Summary: We currently allow you to launch abstract daemons; use
setConcreteOnly() to only list/launch concrete daemons.

Test Plan: Ran "phd list" (no abstract daemons listed), "phd launch
PhabricatorRepositoryCommitDiscoveryDaemon" (reasonable error message).

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T801

Differential Revision: https://secure.phabricator.com/D1487