Remove PyOpenSSL backends (except for openssl_pkcs12) (#273)

* Remove Ubuntu 16.04 (Xenial Xerus) from CI.

* Removing PyOpenSSL backend from everywhere but openssl_pkcs12.

* Remove PyOpenSSL support from module_utils that's not needed for openssl_pkcs12.

* Add changelog fragment.

tests/integration/targets/get_certificate/meta/main.yml

@@ -1,4 +1,3 @@
 dependencies:
   - setup_openssl
-  - setup_pyopenssl
   - prepare_http_tests

tests/integration/targets/get_certificate/tasks/main.yml

@@ -30,17 +30,7 @@
       that:
         - result is success or skip_tests
 
-  when: |
-    pyopenssl_version.stdout is version('0.15', '>=') or
-    cryptography_version.stdout is version('1.6', '>=')
-
-- block:
-
-  - include_tasks: ../tests/validate.yml
-    vars:
-      select_crypto_backend: pyopenssl
-
-  when: pyopenssl_version.stdout is version('0.15', '>=') and not skip_tests
+  when: cryptography_version.stdout is version('1.6', '>=')
 
 - block: