ACME modules refactor (#187)

* Move acme.py to acme/__init__.py to prepare splitup.

* Began moving generic code out.

* Creating backends.

* Update unit tests.

* Move remaining new code out.

* Use new interface.

* Rewrite module init code.

* Add changelog.

* Add BackendException for crypto backend errors.

* Improve / uniformize ACME error reporting.

* Create ACMELegacyAccount for backwards compatibility.

* Split up ACMEAccount into ACMEClient and ACMEAccount.

* Move get_keyauthorization into module_utils.acme.challenges.

* Improve error handling.

* Move challenge and authorization handling code into module_utils.

* Add split_identifier helper.

* Move order code into module_utils.

* Move ACME v2 certificate handling code to module_utils.

* Fix/move ACME v1 certificate retrieval to module_utils as well.

* Refactor alternate chain handling code by splitting it up into simpler functions.

* Make chain matcher creation part of backend.

tests/unit/plugins/module_utils/acme/test_io.py

@@ -0,0 +1,29 @@
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+
+from mock import MagicMock
+
+
+from ansible_collections.community.crypto.plugins.module_utils.acme.io import (
+    read_file,
+    write_file,
+)
+
+
+TEST_TEXT = r"""1234
+5678"""
+
+
+def test_read_file(tmpdir):
+    fn = tmpdir / 'test.txt'
+    fn.write(TEST_TEXT)
+    assert read_file(str(fn), 't') == TEST_TEXT
+    assert read_file(str(fn), 'b') == TEST_TEXT.encode('utf-8')
+
+
+def test_write_file(tmpdir):
+    fn = tmpdir / 'test.txt'
+    module = MagicMock()
+    write_file(module, str(fn), TEST_TEXT.encode('utf-8'))
+    assert fn.read() == TEST_TEXT