Add the choice of interface for keystoneclient

Add the choice of interface for keystoneclient

1. When functest is running on external host not jumpserver,
   keystoneclient is used to retrieve or create projects/tenants
   and users, which would fail to establish connection to the admin
   endpoint.
2. Keystoneclient will connect to the public endpoint by setting the
   env OS_INTERFACE to be public.
3. Remove the admin endpoint check in check_os.sh.

JIRA: FUNCTEST-801

Change-Id: I324f5c8fdf61447319070a23f6b6bc6b1bbc1d48
Signed-off-by: Linda Wang <wangwulin@huawei.com>