Allow users to assign APIs to a specific nodegroup

[vishalbollu]

• Update API spec
• Validate that specified nodegroup is exists
• Validate that a replica of the submitted API can be scheduled onto the specified nodegroup (current validations check across all nodegroups)
• Update documentation

[vishalbollu]

https://app.zenhub.com/workspaces/cortex-5fbdec008baa5b0010888837/issues/cortexlabs/cortex/970 can be addressed with this feature