Actually allow for overriding the lock file

The documentation says that it should be possible and the code already
consults that config key, so relax the validation a bit so that it could
be overridden.

Change-Id: Ic4e8964e4ad4027168e78aef5daf42788e273fa4

boartty/config.py

@@ -45,6 +45,7 @@ class ConfigSchema(object):
               'ssl-ca-path': str,
               'dburi': str,
               'log-file': str,
+              'lock-file': str,
               'socket': str,
               'auth-type': v.Any('basic', 'digest', 'form'),
               }