SSH-related unit tests often involve shelling out to rbssh, which, as of

a recent change in the un-released 3.0.18, now looks up the proper

backend to use in the database. However, since rbssh is its own process,

it doesn't have access to the in-memory test database that the rest of

Review Board is running in, and will pick up the storage backend

configured in the developer's database. This can cause unit tests to

fail, since the environment is set up incorrectly for those tests.
This change introduces a workaround, which is a new

$RBSSH_STORAGE_BACKEND environment variable for rbssh that will take

precedence over any other configured backends. This value, if provided,

will be passed in to SSHClient using the new storage_backend

parameter, and that will short-circuit the storage loading.
Review Board does not set this during normal usage. It only sets it

during unit test runs. However, it might be worth exploring setting this

in a future change, in order to reduce rbssh startup time, but that will

be a separate project.
This also provides a better, more consistent environment variable name

for debugging rbssh, called $RBSSH_DEBUG. Both this and the old

$DEBUG_RBSSH are supported.

Unit tests pass with and without Power Pack's SSHDB enabled locally.
Verified that rbssh from within Review Board and on the command line

was respecting the configured SSH storage backend.