Property group

In heat resource plugins, properties are getting defined with validation schema and there is no group concept exist which is required for following reasons:

* some time, resource mandates to provide either PropertyA or PropertyB should be provided, and one of them is mandatory one for resource. This can't be defined now in heat, as developer can't set required=true for both the properties and as part of validate() method, developer should implement the logic whether one of these property is provided. Such a case occurs in many Resources. This can be over come by:

PropertyGroupA = {
required=true,
PropertyA= {},
PropertyB={},
validation: PropertyA OR PropertyB
}
Here 'validation' define binary operator among them. Like (A AND B) OR (A AND C) AND (B OR E).

PropertyGroup can have child PropertyGroup as well and 'validation' can operate on top of PropertyGroup.

* some of the Plugin supports multiple versions of its thing, for example, docker plugin supports multiple versions. so Some properties are only supported in some versions only. so PropertyGroup can be used with 'plugin_supported_since' as follows:

PropertyGroupA = {
PropertyA = {}
PropertyB ={}
client_plugin_supported_since = 1.2
}

Heat engine can infer that this set of properties in the PropertyGroups are supported only from 1.2 version. so it can check the current client_plugin_version and validate accordingly.