Fix messages in rally exceptions

Many exceptions in rally.exceptions contain "%(message)s" in their msg_fmt
attribute. When raised with message="...", however, these Exceptions just
assign the passed string to the message attribute instead of formatting it
with msg_fmt.

We fix this issue here and also refactor a bit the scenario classes:
* Remove the unused InvalidTaskConfigException;
* Move the DummyScenarioException to the dummy bechmark scenario module;
* Move the Tempest* exceptions to the corresponding modules

Change-Id: I551f38edd5ec7bc67dd952a2203508f64c4b2518
Merge "Fix messages in rally exceptions"