Enable running tests without authorizer.

We do not support creating Master instance without an Authorizer in tests: https://github.com/apache/mesos/blob/aa497e81c945677c570484a8aa1a8c8b2e979dfd/src/tests/cluster.cpp#L217. This leads to a segfault when masterFlags.acls = None(); is used in a test, while it's a valid use case and should be allowed.

Alternatively, we use masterFlags.acls = ACLs();, which triggers creation of LocalAuthorizer with emtpy ACLs, which seems to be semantically equal to the absence of an authorizer, given permissive flag is true. This equivalence should be verified by a test.