Add a block device for the image when using BDMv2

Right now the cli was already adding a block device when it was passed
an image_id to boot from and more than one block device mapping v2. This
is done because nova expects the image to be another block device
mapping and would ignore it otherwise.

This patch moves this functionality from the cli to the base module so
users of the module can benefit from this and also to prevent some
misunderstandings that may arise when using BDMv2 and the image gets
ignored by nova. In the future we should handle this in the nova side
assuming a new BDM when an image is provided.

Fixes bug #1225061

Change-Id: I29f31c24f958cfa8b68b33edc63e0d7031aa241f
Modify --num-instances flag description to clarify limit upper bound

Closes Bug:#1220703

Modified --num-instances flag description in shell.py to clarify that
number of instances spawned are limited by the quota.

Change-Id: I29fa175f310ab1e073dca5f453a3540dcdde933d
Signed-off-by: Abhishek Lahiri <aviostack@gmail.com>
Merge "Add a block device for the image when using BDMv2"
Merge "Modify --num-instances flag description to clarify limit upper bound"