Fix maas_horizon_scheme and maas_heat_cfn_scheme variables

Duplicate entries for "scheme" on horizon and heat_api_cfn mean that
this can't be overriden like the others.

* Remove "scheme: {{ maas_scheme }}" lines for both horizon and
  heat_api_cfn
* Ensures the overrides are possible

Fixes #36
Merge pull request #42 from andymcc/maas_scheme_fix

Fix maas_horizon_scheme and maas_heat_cfn_scheme variables