Many versions of Review Board currently have a bug where sending an

unknown value in the tool= parameter to the repository list API would

cause a crash. We'll be shipping a fix for that, but unfortunately there

are a variety of released versions where the Git and ClearCase clients

can trigger this crash on servers that do not have Power Pack installed.
This change fixes the RBTools side to not include the Power Pack SCMTool

names, which will avoid the problem for existing servers. For new

servers, we'll be shipping a server-side fix that fixes the crash, adds

the SCMTool IDs to the capabilities, and allows us to pass SCMTool IDs

rather than names to the repository list API. If we see the IDs in the

capability blob, we can assume that we can pass the IDs, including

potentially unknown IDs.
This also fixes a problem where we were sometimes accessing the

repository list API twice with exactly the same parameters.

Ran unit tests.
Verified that the repository list API was accessed using SCMTool names

  that did not include potentially missing ones when running against an

  older server.
Verified that the repository list API was accessed with SCMTool IDs

  when running against a server with the new API fixes.