This introduces set_ssh_storage_backend_path() and

get_ssh_storage_backend_path(), which are wrappers around the

ssh_storage_backend siteconfig setting. Right now, these don't do a

whole lot, but they give us a standard way for extensions to set a new

backend, and eventually can let us better apply this to the environment

for rbssh (which is not planned for 3.0.x, as it's a more involved

project).
Extensions will also be able to check for the existence of these

functions in order to determine whether to set the old

settings.RBSSH_STORAGE_BACKEND or to set the new key in siteconfig.

Tested these with a patch to Power Pack.