[vbox] Increasing memory/disk size in vbox scripts.

Closes-bug #1253097

When controller is deployed with Ceph,
768Mb of RAM is not enough. So we have
to increase the default amount of memory
required to run demo environment.
Also, as we now give more space on
controllers for root partition, increased
default disk size as well.

Change-Id: I791f26c94c54ce6be48116fc2eaa9e6464da5738