Introduce exometer metrics into 2.0

Starts Exometer in order to avoid noproc errors for exometer_admin when
running EQC tests

Adds Exometer startup for the EQC tests and removes startup/shutdown of
the riak_core_stat_cache which is no longer used

Removes startup of riak_core_stat_cache from the EQC tests

Bump eunit_formatters so it doesn't have the badarith problem.

Fix failures in get_fsm_eqc.

1) We are using exometer now, and so starting and updating a stat in
   folsom is pointless; create and update the stat using exometer
   instead. This was not contributing to the failure necessarily.

2) Start the riak_core_ring_manager in the fsm_eqc_util.

   The get FSM may call `riak_core_bucket:get_bucket/1` on a plain
   bucket when checking for the `dvv_enabled` property. Because for
   untyped buckets this checks the public ETS table created by the ring
   manager and not cluster metadata, the table must exist or you get a
   badarg from `ets:lookup/2`. Starting the ring manager in `test` mode
   is sufficient to prevent this error.