engine: Introduce commands to manage iscsi bonds entities

These commands allow to create, update and delete iscsi bonds in the
scope of the data center. They perform basic validations, provide audit
logs and database manipulations like adding a logical network or/and
iscsi storage targets to the iscsi bond or removing them from the bond.
Commands don't update VDSM about changes in this patch.

Change-Id: I06890ea40d22b86d9421cb1b0c68ea28bc430b0a
Bug-Url: https://bugzilla.redhat.com/753541
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>