[VNC API server] Improve cassandra object_read method
[VNC API server] Improve cassandra object_read method

Use pycassa 'multiget' method when it's possible.
Two cases can be distinguished:
- cannot determine return size when fetching multi keys with start and
  finish column conditions. In that case try to fetch data with
  multiget and if it raises 'OverflowError' try to fetch data with
  'xget' method.
- when the column length is defined, we calculate if we can fetch all
  data in one 'multiget' call, if not use all necessary 'get' calls

Change-Id: I2497ea365e25e7cfa6abd8c35b36929f7b5e7806
Partial-Bug: #1589014

rombie/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,codilime/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,codilime/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller