taskotron.yaml: provide a default value for fas_password
    
    When testing Bodhi code with fake_fedorainfra, some FAS password must be
    provided (can't be empty), the bodhi API does not allow it. Providing
    some (any non-empty) value makes it easier for the developer - one less
    option to configure, one less problem to debug in case he forgets. This
    does not affect regular dev workflow (without custom Bodhi) in any way,
    and in production it has to be configured anyway.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D423