add servicetemplate validate method to OrchestratorPlugin interface

[swaroopar]

• [ ] Add a validateServiceTemplate method to OrchestratorPlugin. Returns boolean.
• [ ] For Huaweicloud plugin - validate the regions in the template with the values available using IamRegion
• [ ] For Flexible Engine - Check if similar class is available. Otherwise no validation necessary. Just return true.
• [ ] All other cloud providers - return true.