fix(snippet-file-name): removed appending of ".conf" for snippets files

[sticky-note]

Since introduction of TOFS in this formula, we can retrieve file that are not described by pillar. TOFS will generate source like the following:

nginx_snippet_letsencrypt:
              source:
                  - salt://nginx/files/snippets/letsencrypt
                  - salt://nginx/files/snippets/server.conf
                  - salt://nginx/files/A/B/C/letsencrypt
                  - salt://nginx/files/A/B/C/server.conf
                  - salt://nginx/files/FreeBSD/letsencrypt
                  - salt://nginx/files/FreeBSD/server.conf
                  - salt://nginx/files/default/letsencrypt
                  - salt://nginx/files/default/server.conf

However, we have not same behavior between snippets file and server_configs.
Actually formula, append .conf as an extension for all snippets.
If we include snippets/*.conf in the main nginx.conf for example, it is not always the case that we desire to include all snippets.
This PR propose to remove the appending of '.conf' for snippets files and permit to choose file name and extension for snippets with snippet name.

[noelmcloughlin]

Thanks @sticky-note
I cannot see any technical issues with the code.
Could you update the PR description to elaborate on what problem is being fixed?

[daks]

Hello, I think this PR introduces a 'BREAKING CHANGE' because people must adapt their pillar. This must be explicitely stated in the commit message, can you update it?

[sticky-note]

@daks @noelmcloughlin updated, is that ok?

[daks]

Not sure the commit 'BREAKING CHANGE' message is formatted correctly https://github.com/saltstack-formulas/nginx-formula/blob/master/docs/CONTRIBUTING.rst#use-breaking-change-to-trigger-a-major-version-change

otherwise good for me, nice catch @sticky-note :)

[sticky-note]

@daks If I read correctly, it is ok now. Sorry for multiple editing.

[noelmcloughlin]

Thanks @sticky-note for the hard work and responsiveness & @daks for reviewing

[myii]

Thanks to everyone for this PR. For reference, this PR links back to #238 (comment).

[saltstack-formulas-travis]

🎉 This PR is included in version 2.0.0 🎉

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