When running some rbt commands outside of a repository (such as

rbt api-get or rbt patch --print) and using the TREES

configuration to define the server URL, rbtools would exit with a

message about being unable to find a repository. This was a regression

caused by the previous fix in 5.2 to allow these commands to use TREES.
This fix plumbs through the tool_required flag to

scan_usable_client, making it so we don't exit if we don't find a tool

at all.

Ran unit tests.
Was able to successfully use rbt patch --print, rbt patch --write,

  and rbt api-get outside of any source tree when using TREES to

  define the server URL.
Verified that TREES still worked as expected.