stack/erp5: stop using caucase managed certificate

In nexedi/slapos@74d18b9d I switched ERP5 to use caucase certificate, but caucase is not yet usable for this and this caused issues with upgrading existing instances.

We'll maybe try caucase again here, in that case 620c9332 can be reverted if we want to reuse some parts of what was made.

See merge request nexedi/slapos!857