restapi: Add REST API support for iSCSI multipathing

Enable getting, creating, updating and removing iSCSI Bonds through REST
API including adding/removing a storage target (Engine term is storage
connection is a little bit misleading) and adding/removing a logical
network.
In case of storage target and network the "add" acts more like
"attach" because its adding an already existing target or/and network to
the iscsi bond.
Usages:
Creating a new iSCSI Bond:
POST /api/datacenters/{datacenter_id}/iscsibonds HTTP/1.1
Accept: application/xml
Content-type: application/xml
<iscsi_bond>
  <name>fromRest</name>
  <storage_connections>
    <storage_connection id={storageconnection_id} />
    ....
  </storage_connections>
  <networks>
    <network id={network_id} />
    ....
  </networks>
</iscsi_bond>
Updating an iSCSI Bond, only name and description can be editing that
way:
PUT /api/datacenters/{datacenter_id}/iscsibonds/{iscsibond_id}  HTTP/1.1
Accept: application/xml
Content-type: application/xml
<iscsi_bond>
        <name>{name}</name>
        <description>{description}</description>
</iscsi_bond>
Removing an iSCSI Bond:
DELETE /api/datacenters/{datacenter_id}/iscsibonds/{iscsibond_id}
HTTP/1.1
Getting all iSCSI Bonds for the specified datacenter:
GET /api/datacenters/{datacenter_id}/iscsibonds HTTP/1.1
Accept: application/xml
Content-type: application/xml

Change-Id: Id8881110cbeb163e9fc09e98bf4497d894f40490
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>