Add list_items method to Catalog OM

Change-Id: I2528516b07db12d2f2e2375e4b0efe9c12130a39
Add list_vms to VApp OM

Change-Id: I67d4bed2768846a668f7c3ba1ce85d176105cde7
Add list_networks method to VDC OM

Change-Id: I217552a71554162a275b2772c78f2bf31b4198e1