api: Fix approve API to allow deploy with public key

Currently we don't set the authentication method and engine searches for
password validation. Approve API should use PK auth as default except
password and authentication method are set.
(Apperently approve call by API was never tested and currently fails
on "empty password")

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1101565
Change-Id: I9816d907d45ec7b4ee066fa369ca1150e688084f
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>