setup-ssh: Also use keys from the ssh-agent

Currently, setup-ssh only uses one disk-based key. This means that any
setup where we use keys from ssh-agent (which do not necessarily exist
on disk) will break when moving from the old method to setup-ssh.

This patch moves the SSH key handling to separate functions, and uses
both the disk key (first) and the agent keys for login.

The patch also fixes the root_logger setup level (I tried to hard to
reduce noise and broke the debug level, sorry).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>