kubernetes: Add Plugin Object support.

kubernetes: Add Plugin Object support.

This patch adds PluginObject support to the Kubernetes Backend.
Backup job will create two Plugin Objects:
1. aggregated number of PODs backuped
2. aggregated number and size of PVCs backuped
I.e.
*llist objects objectid=59
       objectid: 59
          jobid: 2,212
           path: /@kubernetes/
       filename:
     pluginname: kubernetes: ns=plugintest debug verify_ssl=0
 objectcategory: Container
     objecttype: POD
     objectname: Kubernetes PODs
   objectsource: 192.168.2.140:6443
     objectuuid: 192.168.2.140:6443
     objectsize: 0
   objectstatus: U
    objectcount: 8

*llist objects objectid=60
       objectid: 60
          jobid: 2,212
           path: /@kubernetes/
       filename:
     pluginname: kubernetes: ns=plugintest debug verify_ssl=0
 objectcategory: Container
     objecttype: PVC
     objectname: Kubernetes Persistent Volume Claims
   objectsource: 192.168.2.140:6443
     objectuuid: 192.168.2.140:6443
     objectsize: 5,368,709,120
   objectstatus: U
    objectcount: 5