Switch to os-brick encryptor classes

This blueprint aims to drop the encryptor classes and supporting code from the
Nova codebase in favor of the classes provided by os-brick. This is made
possible by the following os-brick change that introduced new encryption
provider constants during Ocata :

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Thanks to the following bugfix also released for Ocata the constants
present in os-brick also support the use of the deprecated legacy class
paths from Nova, for example nova.volume.encryptors.luks.LuksEncryptor,
while using the os-brick provided classes :

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