masher: De-hardcode i386 when possible

EPEL7 doesn't have this architecture, so we shouldn't hardcode it if
possible.

This helps with #86
masher: Allow per-repo arches configuration

This is important for EPEL 7, which doesn't have i386, whereas all other
releases have it.

Fixes #86
Merge pull request #88 from fedora-infra/feature/epel7-again

Two more changes helping with EPEL 7