broker, agent: more robust service startup/shutdown

The agent and broker had no ordering, and there was no way for an
external program to reliably tell when the broker was ready to
serve requests.

With this change:
- The agent and broker services gain an ordering dependency between
  them in systemd.
- The broker attempts to clean its socket file upon exit, so that the
  existence of the file generally indicates that the broker is ready
  to serve requests.
- The agent will be more patient in attempting to connect to the broker.

Change-Id: I392ddf54503d7b9ef3448d22385b547222c0cc86
Bug-Url: https://bugzilla.redhat.com/1018829
Signed-off-by: Greg Padgett <gpadgett@redhat.com>