Make it easy to use the e2e tests to test upgrading a running master

by adding support for testing official release versions and writing a simple script to first bring up and test the old version before upgrading it and testing it again. Related to supporting in-place upgrades of masters (#2524).

The downsides of this are that by default it uses the e2e tests from HEAD, which may not be expected to pass on an old cluster, and that it uses the client binaries from head. I can send a follow-up to fix the client binaries issue, but don't have anything planned for supporting old e2e tests.

I'm not sure how our Jenkins setup is configured, but it'd be nice to get something along these lines added to it in the future.
Switched to require the 'v' in the version string and clarified the description.

I think it makes sense to keep version and build separate for the sake of clarity, but I've changed the default value of version from "build" to "" and removed the special case, since doesn't seem any more understandable.