This was marked optional in change If57aa3e37 but it has
only been optional since Queens due to change I8a3e7e6c4,
before that you will get a 400 error if you do not provide
boot_index in the BDMs, e.g.:
2018-04-23 12:34:13,308 INFO [nova.api.openstack.wsgi] \
HTTP exception thrown: Block Device Mapping is Invalid: \
Boot sequence for the instance and image/block device \
mapping combination is not valid.
2018-04-23 12:34:13,310 INFO [nova.api.openstack.requestlog] \
127.0.0.1 "POST /v2.1/6f70656e737461636b20342065766572/servers"
status: 400 len: 164 microversion: 2.1 time: 0.129485
One could argue that I8a3e7e6c4b72eb1c3707d54049d18dc29f606fe5
is a behavior change that should have gone with a microversion,
which if people agree with that, could be reverted separately.
Change-Id: I14d44dbc0b6a8fb25932c333e695cad9edaefbed
Closes-Bug: #1766306
048fa23100