Use parameter constraints for image, key and flavor

Use parameter constraints for image, key and flavor

If you don't have (or provide) the wrong image, KeyName,
or flavor, we fail at some later point (not always early,
depending on what's wrong).

Since Icehouse, Heat has had a "custom constraints" method
of dynamically validating parameter values, by comparing the
value provided with a list from the underlying service.

Despite the name, there's nothing "custom" about the constraints,
these ones are included in Heat by default (though they are pluggable,
which is where the name comes from..)

See the docs for more info:
http://docs.openstack.org/developer/heat/template_guide/hot_spec.html#custom-constraint

Note, I've not considered network validation here, this could
possibly be added in a subsequent patch.

These constraints are evaluated via any of the following:
- heat template-validate -f <template>
- heat stack-preview <arguments given to create>
- heat stack-create <arguments, fails fast before creating anything>
- heat stack-update <arguments, fails fast before updating anything>

Change-Id: I3a6374ce5421575cdde893c62aa97c750a07acd8