Add ExternalID parameter to TabletExternallyReparented.

This ID can optionally be provided by the external tool doing the
reparnt. The ID gets printed in logs to allow cross-referencing with
logs from the external tool.
Merge pull request #161 from youtube/tablet-externally-reparented

Add ExternalID parameter to TabletExternallyReparented.
Make topo server flavor pluggable in integration tests.
Remove extra copy of zk_setup().
Merge pull request #162 from youtube/topo-server-flavor

Make topo server flavor pluggable in integration tests.
Centralize common test options in one place.

There are now three different main() functions in our test suite.
Let's have them all get common options from one place, so we don't
have to remember to add new options everywhere.
Merge pull request #163 from youtube/test-options

Centralize common test options in one place.