MyBS.pm: support the password-store keyring
    
    osc can use password-store via the Python keyring and password-store backend.
    We can detect this configuration from its specific credentials_mgr_class
    setting, and instead call the 'pass' command directly, similarly to the
    secret-tool.
    
    Signed-off-by: Vlastimil Babka <vbabka@suse.cz>