Stable download locations

[benmaddison]

The current practise of keeping recent releases in tools/rpki/validator3/prod/generic/ and thereafter moving them to tools/rpki/validator3/archive/generic/ makes constructing automation routines for retrieval and installation unnecessarily hard.

It would be helpful if all releases were published at a single stable url, and then left there forever (for some reasonable value of "forever")

[zzaflemi]

I feel the same way. Then you could also do things like yum downgrade as far as the CentOS packages go. (Not withstanding likely needing to remove the rpki DB and letting it rebuild.) I brought this up in the past, but was simply told there is an archive I can go get old builds from.

#80