As discussed in Berlin Summit, there are some
considerations should be added to the design:
1. Expose ``boot_index`` in this feature could
make it easier for users to detach and attach
volumes[1].
2. As discussed in Maillist[2], we are not going
to allow detach/attach volumes for instance in
states other than STOPPED,SHELVED/SHELVE_OFFLOADED
so we make it clear in the spec.
[1] https://etherpad.openstack.org/p/BER-bfv-improvements
[2] http://lists.openstack.org/pipermail/openstack-discuss/2019-January/001344.html
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