ancestry_path is deprecated; use ancestors

[yukinying]

https://github.com/forseti-security/config-validator/blob/87b4ae546420814b2a6766a1b4569278f5e1627e/api/validator.proto#L44

  // Ancestor list as returned by CAI (added sometime around Oct 2019)
  repeated string ancestors = 6;

I have also verified that CAI now do not return ancestry_path. Instead it returns ancestors, which is a list of strings instead of a single string. For example,

		"ancestors": [
			"folders/12345",
			"organizations/67890"
		]

I think this will make the logic on generating ancestry be easier.

[melinath]

It looks like config-validator uses ancestry_path internally and converts "ancestors" to a path if it's provided - however, I think this would make the expected structure more clear to users and it would be easier to change now than later.