Allow `auth.get_user()` method to be called multiple times.
The `ImportModuleAPI` calls the `auth.get_user()` which auths the
user using Kerberos. the `ImportModuleAPI` later calls `SCMHandler`
which in its `__init__` method calls the `auth.get_user()` again.
This leads to traceback in GSSAPI, because the user is already
authed.
This commit fixes this by caching the auth results in `flask.g`,
which is reset after each request based on the Note in
http://flask.pocoo.org/docs/1.0/appcontext/#storing-data.
This commit also marks mutual auth as OPTIONAL in `mbs-cli`,
because MBS server currently does not do mutual auth.