[devel] Get binary tags from SoftConfDB

For steps which have SystemConfig unset the SoftConfDB is queried for a list of suitable platforms. I then try all combinations to find the minimum.

Remaining things to change:

• Replace XML RPC with something that is more resilient
• Modify LbPlatformUtils.lowest_common_requirement to support multiple platforms per step
• Return haswell-any for combinations which cannot run on the same host, e.g. x86_64-slc5-gcc49-opt and x86_64+fma-centos7-gcc7-opt

These won't change the functionality so it might make sense to merge first so validation can start.