Ec2MultiRegion Support

# Summary

The followings are added to enable Cassandra instances launching with Ec2MultiRegion support.

# Options

```
--multiregion
  To enable multiregion support.  Values: true.
  REQUIRED
  e.g. --multiregion true --clustername t-cluster --version community --totalnodes 2

--seeds 122.33.44.55,33.44,55.66,111.222,333 
  When creating a new cluster to join an existing cluster, the public IPs of the existing cluster must be specified.
  REQUIRED (only if you are joining an existing cluster)

--opscenterip 122.33.44.55
  When creating a new cluster to join an existing cluster, the public IP of the opscenter in the existing cluster.
  REQUIRED (only if you are joining an existing cluster)

  e.g. --multiregion true --clustername t-cluster --version community --totalnodes 2 \
         --seeds 122.33.44.55,33.44,55.66,111.222,333 --opscenter 122.33.44.55

```

# Note
Sometimes it may take a few minutes for the opscenter to reflect the new datastax agents joining an existing cluster.