launch multiple instance with boot volume

[Andy Chong]

launch instance form should block if user tries to launch multiple instance with volume or volume snapshot
doing so will only ends up with one instance boot up successfully while the rest end up in error

steps to reproduce:
create one volume
launch at least 3 instance with the volume created above
see all the instance fails at task 'Block_Device_Mapping' except one instance

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/5668

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/5668
Committed: http://github.com/openstack/horizon/commit/599f7f50dde52cae88a958ec3e78ff31f70842c0
Submitter: Jenkins
Branch: master

commit 599f7f50dde52cae88a958ec3e78ff31f70842c0
Author: Andy Chong <email address hidden>
Date: Thu Mar 22 15:02:33 2012 +0800

    Block launching more than one instance if volume is specified

     * fix bug 961866

    Change-Id: I5e25d38f67d56268e9e3e904df7a4d873ecb98d5