tools: Fixes empty provider option when editing domain

Fixes error during engine-manage-domains edit action when provider is
not entered.

Change-Id: I79effa66047579b0bc5aa8772a6e90dc849a4cd3
Bug-Url: https://bugzilla.redhat.com/1055417
Signed-off-by: Martin Perina <mperina@redhat.com>