Remove reference to ssh::bastion

That class was removed in https://gerrit.wikimedia.org/r/#/c/90098/

It barely did anything, but there were still a few references.  This
patch replaces those references with an explicit install
of mosh, which was the only remaining thing that class did anyway.

Change-Id: I08fc1c6d8a9de4d37cda61673f07ce183b2ce793