1. Changed ModuleBuild's context property to db column, it's
non-nullable and default value is '00000000' to keep it consistent
with previous behaviour.
2. Changed ModuleBuild.contexts_from_mmd to return a tuple of
(ref_build_context, build_context, runtime_context, context).
3. Updated tests affected by this change.
koji now supports tags with max length of 256, we can use
more informative tag name instead of the hash one.
The new format of koji tag name is:
module-<name>-<stream>-<version>-<context>
However when the generated tag's length > (256 - len('build')), we
fallback to the old way of name in hash format (module-<hash>).
In this change, koji tag is always generated from MBS itself, even
with pdc resolver.
FIXES: #918#925
