qa/tasks/keystone: use "keystone-manage bootstrap"

qa/tasks/keystone: use "keystone-manage bootstrap"

* qa/tasks/keystone.py:
  instead of prefilling keystone manually, use "keystone-manage bootstrap"
  instead. it helps to setup the admin user, a "Default" domain with
  "default" id, and wire them up with the expected role and a "admin" project,
  etc. as id of the admin domain is known to be "default", we can just use it
  in our tests without querying openstack for the id of "Default"
  domain. this is very handy.
* qa/suites/rgw/tempest/tasks/rgw_tempest.yaml:
  use "Default" for domain name. as "Default" is the name of the domain
  created by bootstrap, while "default" is its id.
* qa/suites/rgw/crypt/2-kms/barbican.yaml:
  remove settings to bootstrap keystone

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