extra_cluster_params should merge dictionaries

[coyotemarin]

Currently, if you do something like this in your config:

runners:
  emr:
    extra_cluster_params:
      Instances:
        EmrManagedMasterSecurityGroup: sg-00000000000000000

it'll actually blow away the entire Instances parameter to RunJobFlow, rather than just setting the security group. This is almost certainly not what users expect.