rename DIEGO_BETA -> DIEGO_STAGE_BETA [#80165358]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
Finish the rename s/CF_DIEGO/DIEGO/g

Staging on Diego is now controlled by DIEGO_STAGE_BETA=true
Running on Diego is now controlled by DIEGO_RUN_BETA=true

[#80165358]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>