Add utility functions for working with the default SSH storage backend.
Review Request #11008 — Created May 5, 2020 and submitted
get_ssh_storage_backend_path(), which are wrappers around the
ssh_storage_backendsiteconfig 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
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_BACKENDor to set the new key in siteconfig.
Tested these with a patch to Power Pack.