Block Storage API V3 (CURRENT) in cinder - remove Optional flag for key_size and cipher

Regarding https://bugs.launchpad.net/cinder/+bug/1926630 - Creating an
encrypted volume type without a cipher accepted but will fail to create
an encrypted volume - we should update the documentation.

For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
(e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode). You
can do 64 bit (e.g. blowfish-64 with CBC), but anything below 128 bit
has to be considered insecure today. Maybe we can set a default to 256.

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-----------------------------------
Release: on 2021-05-25 19:08:09
SHA: da2fbff247f23c7b82e77db2c1e7772cb22801ac
Source: https://opendev.org/openstack/cinder/src/api-ref/source/v3/index.rst
URL: https://docs.openstack.org/api-ref/block-storage/v3/index.html?expanded=create-an-encryption-type-detail