change_hat using probing to find and transition to the first available
hat. Hats missing as part of this probe are expected and should not
be logged except in complain mode.

BugLink: http://bugs.launchpad.net/bugs/1615893Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>