restapi:Fix CRUD operations for iSCSI Storage connection.

1) renaming class BackendIscsiBondStorageServerConnection to
BackendIscsiBondStorageConnectionResource to be compatible with the
plural resource name
2) Fix remove and get operations for Storage Connection in IscsiBond
3) Adding appropriate tests for resource commands in REST.

Usages:
Get a list of Storage Connections contained in the iSCSI bond:
Get
/api/datacenters/{datacenter_id}/iscsibonds/{iscsibond_id}/storageconnections/

Get a specific Storage Connection in the iSCSI bond:
Get
/api/datacenters/{datacenter_id}/iscsibonds/{iscsibond_id}/storageconnections/{storage_id}/

Adding a new Storage Connection to an existing iSCSI Bond:
POST /api/datacenters/{datacenter_id}/iscsibonds HTTP/1.1
Accept: application/xml
<storage_connection id="{connection_id}"></storage_connection>

Remove a Storage Connection from the existing iSCSi Bond:
DELETE /api/datacenters/{datacenter_id}/iscsibonds/{iscsibond_id}/storageconnections/{storage_id}/ HTTP/1.1

Change-Id: I2424067b4371b6a99f61ea687155c4082855aff5
Bug-Url: https://bugzilla.redhat.com/1133896
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>