Add sharding support to multiproduct_kati

This is so that we can split the build_test build on large branches into
multiple builds, each testing a subset of the products.

Test: m blueprint_tools  (runs the go unit tests)
Test: build/soong/build_test.bash --only-config
      vs
Test: build/soong/build_test.bash --only-config --shard-count=2
Test: build/soong/build_test.bash --only-config --shard-count=2 --shard=2

Change-Id: I40ccc1aa477bc0ffa74ff564d155068509be18f0