sshd only asks exactly once for the password. C.f. upstream issue
SSHD-850.[1] So we have to work around this limitation for now.
Once we move to sshd > 2.1.0, this can be simplified somewhat.
[1] https://issues.apache.org/jira/browse/SSHD-850
Bug: 520927
Change-Id: Id65650228486c5ed30affa9c62eac982e01ae207
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>