test/mgr: restructure dashboard tests

test/mgr: restructure dashboard tests

* restructure dashboard tests so they follow the model of other tests
  based on ceph-helpers.sh. i.e. extract the TEST_* out of run()
  function.
* expose mon-host using CEPH_ARGS env variable.
* mgr-dashboard_v2-smoke.sh: do not export CEPH_ARGS in a subshell, so
  the run_mgr() and ceph cli can also have access to the updated
  CEPH_ARGS, and hence they will be able to locate the mon. and because
  auth-supported=none is populated using CEPH_ARGS, this test won't fail
  because of missing keyring.

Signed-off-by: Kefu Chai <kchai@redhat.com>