Refactor `MesosTest::StartSlave` in favour of builder style interface

Every overload of `MesosTest::StartSlave` method depend on `cluster::Slave::create` method, which accepts several arguments. In fact, each overload of `MesosTest::StartSlave` accepts a subset of combination of arguments that `cluster::Slave::create` accept. Given that the latter accepts 11 arguments at the moment, and there are already 13 overloads of the `MesosTest::StartSlave`, introducing a builder-style interface is very desirable. It'd allow adding more arguments to the `cluster::Slave::create` without the necessity to update all existing overloads. It would be a local change as it won't affect existing tests.

See this comment.