teuthology: remove previous test ssh keys

Updated the ssh-keys task to cleanup
any left-over keys from previous tasks
(indicated by the user being 'ssh-keys-user').
Also, some of the functions in the ssh_keys task seem
like they could be useful in general.
This patch refactors them into misc.py.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewd-by: Sam Lang <sam.lang@inktank.com>