The `CephBlockPool` CR allowed any algorithm to be specified in
`spec.erasureCoded.algorithm`, but in practice only `isa` or `jerasure`
would work.
Make that restriction explicit by restricting this field to `isa` or
`jerasure` in the CRD.
Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>
(cherry picked from commit 0b423a30f9)