This role also sets up authorized_keys so that inter-node ssh works
between the hosts for alternative users.
This role will be useful for things like devstack, which require an
additional user be able to ssh around for things like nova's migrations.
The role can be called multiple times, with a different target user each
time. It is assumed that the user already exists.
Change-Id: I5972d66a48802fce0c1ebb885be00c5803ff0e9a