Add an index column to 'nova list'

Now 'nova list' only shows the 'ID'/'Name'/'Status'/'Task State'/'Power State'/'Networks' columns. That's OK for normal using.

But if you have more than 200 VMs or even larger in one environment,
'nova list' will print out lots of lines even if I add some filters (such as '--name'/'--status'/..).

It's very hard to count the amount of them.
Like this: http://paste.openstack.org/show/60553/

I just have to use 'nova list |wc' to count them (minus 4 line)..

So, if an index column add to 'nova list',
it can easily give the amount of the VMs under filtering opinions('--name'/'--status'/..).