tfengine should output a warning if an IAM policy has been overwritten

[yeweidaniel]

In the projects recipe, under the iam_members section, if a role is listed multiple times, only the last occurrence will take effect. This has caused bugs several times in our tests and is not an obvious behavior. Propose that we add a warning or make the roles cumulative.

[xingao267]

Chatted with Daniel offline. They want it by end of Q1 as well. But lower priority compared to #1083.