Multiple {{role.name}}ExtraGroupVars definitions aren't merged, the latest definition is only valid.

Issue

• Multiple {{role.name}}ExtraGroupVars definitions aren't merged, the latest definition is only valid.
  For example, when you defined ControllerExtraGroupVars at a.yaml and b.yaml,

- a. yaml
parameter_defaults:
  ControllerExtraGroupVars:
    XXXX : aaaa

- b.yaml
parameter_defaults:
  ControllerExtraGroupVars:
    YYYY : bbbb

then it is assigned to overcloud deploy command as follows,

$ openstack overcloud deploy
<snip>
-e a.yaml
-e b.yaml

In this example, the assigned value in b.yaml is only effective.