gnt-instance reinstall: display OS changes

Currently, when reinstalling an instance, especially when selecting an
OS interactively, is hard to be sure one has chosen the correct OS.

This patch adds displaying the selected OS. I'm not entirely happy, as
it's not very good English (e.g. keeping the same OS should be "but
keep the same OS", and changing should be "and also change …") but I
didn't find a better handling of all the cases:

node1# gnt-instance reinstall -o lenny-image+default instance1
This will reinstall the instance 'instance1' (and change the OS to
'lenny-image+default') which removes all data. Continue?
y/[n]/?: n

node1# gnt-instance reinstall instance1
This will reinstall the instance 'instance1' (and keep the same OS)
which removes all data. Continue?
y/[n]/?: n

node1# gnt-instance reinstall instance1 instance2
The reinstall will operate on 2 instances.
Note: this will remove *all* data for the below instances! It will
keep the same OS.
Do you want to continue?

node1# gnt-instance reinstall -o lenny-image+default instance1
instance2
The reinstall will operate on 2 instances.
Note: this will remove *all* data for the below instances! It will
change the OS to 'lenny-image+default'.
Do you want to continue?

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>