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.

plugins/module_utils/acme/backends.py

@@ -0,0 +1,58 @@
+# -*- coding: utf-8 -*-
+
+# Copyright: (c) 2016 Michael Gruener <michael.gruener@chaosmoon.net>
+# Copyright: (c) 2021 Felix Fontein <felix@fontein.de>
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+
+import abc
+
+from ansible.module_utils import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class CryptoBackend(object):
+    def __init__(self, module):
+        self.module = module
+
+    @abc.abstractmethod
+    def parse_key(self, key_file=None, key_content=None):
+        '''
+        Parses an RSA or Elliptic Curve key file in PEM format and returns a pair
+        (error, key_data).
+        '''
+
+    @abc.abstractmethod
+    def sign(self, payload64, protected64, key_data):
+        pass
+
+    @abc.abstractmethod
+    def create_mac_key(self, alg, key):
+        '''Create a MAC key.'''
+
+    @abc.abstractmethod
+    def get_csr_identifiers(self, csr_filename=None, csr_content=None):
+        '''
+        Return a set of requested identifiers (CN and SANs) for the CSR.
+        Each identifier is a pair (type, identifier), where type is either
+        'dns' or 'ip'.
+        '''
+
+    @abc.abstractmethod
+    def get_cert_days(self, cert_filename=None, cert_content=None, now=None):
+        '''
+        Return the days the certificate in cert_filename remains valid and -1
+        if the file was not found. If cert_filename contains more than one
+        certificate, only the first one will be considered.
+
+        If now is not specified, datetime.datetime.now() is used.
+        '''
+
+    @abc.abstractmethod
+    def create_chain_matcher(self, criterium):
+        '''
+        Given a Criterium object, creates a ChainMatcher object.
+        '''