Wipe out existing id_rsa.pub and id_rsa before pushing ssh keys

A very simple change. Just touch a file first (to create it if it
doesn't yet exist so the delete doesn't error out) and then delete
it before pushing the keys to the file. This should avoid the
id_rsa.pub and id_rsa files from getting messed up due to previous
runs which were interrupted or failed (or if those files exist for
some reason). This appears to be what was causing breaking in the
ceph-deploy nightlies.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>