Lower weave RAM settings.

- Since Weave 1.8.x was rewritten in Golang we may decrease RAM settings
  to continue using g1-small for CI

roles/network_plugin/weave/defaults/main.yml

 ---
 # Limits
-weave_memory_limit: 500M
+weave_memory_limit: 400M
 weave_cpu_limit: 30m
-weave_memory_requests: 300M
+weave_memory_requests: 200M
 weave_cpu_requests: 10m