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.
The goal of this client is to allow simple access to endpoints we use
to manage MBS - like "import" module or "retire" module.
It is not intended to contain tasks available in pyrpkg.
So far only "import" module is implemented, because that's the only
command we have REST API for.
