Retrieve particular user's password for a specified credential set from system keyring.

def retrieve_password_from_keyring(credential_id, username):
    '''
    Retrieve particular user's password for a specified credential set from system keyring.
    '''
    try:
        import keyring  # pylint: disable=import-error
        return keyring.get_password(credential_id, username)
    except ImportError:
        log.error('USE_KEYRING configured as a password, but no keyring module is installed')
        return False