Add RTS interfaces

[frisky-dis]

What this PR does / why we need it: This PR adds RTS interfaces.

Special notes for your reviewer: NONE

Release noteNONE

[coveralls]

Pull Request Test Coverage Report for Build 140

• 308 of 742 (41.51%) changed or added relevant lines in 16 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-3.4%) to 62.348%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
openstack/rts/v1/stackresources/results.go	26	28	92.86%
openstack/rts/v1/stacktemplates/results.go	5	9	55.56%
openstack/rts/v1/stacks/errors.go	0	9	0.0%
openstack/rts/v1/stackevents/requests.go	8	17	47.06%
openstack/rts/v1/stackevents/results.go	29	42	69.05%
openstack/rts/v1/stackresources/requests.go	24	51	47.06%
openstack/rts/v1/stacks/utils.go	36	82	43.9%
openstack/rts/v1/stacks/template.go	29	86	33.72%
openstack/rts/v1/stacks/requests.go	68	149	45.64%
openstack/rts/v1/stacks/environment.go	0	83	0.0%

Totals
Change from base Build 128:	-3.4%
Covered Lines:	3335
Relevant Lines:	5349

---

💛 - Coveralls

[Savasw]

@niuzhenguo Can you please help update if the coverall result would be blocker for merge. Also suggest what steps could possibly help resolve it.

[niuzhenguo]

@Savasw That's because tests coverage decreased with the change, you can check the file list above to find which parts missing coverage. I'm not sure if it's a blocker for merge now but we do not care much about that before :P. @freesky-edward can you please help to confirm?

[freesky-edward]

thanks for your contributing, regarding to the coverage rate, it should pass before merging it, but I am not concerned this much. It can go if all is good enough.
I have some confusion about the relationship of TE, template, environment. it looks TE is an abstract struct of other two. if so, I would suggest to separate it from utility file(util.go).

[freesky-edward]

if this is an internal method, I would prefer to name it starting with lowercase letter.

[Savasw]

@freesky-edward As discussed, for RTS Stack, we have followed code architecture from openstack gophercloud where TE is abstract for both environment and template as there are lot of common features that work for both. Also Fetch function is not used externally at the moment but could be useful for end users separately to fetch content from URL.

[freesky-edward]

@Savasw thanks for your response, LGTM