Python interpreter discovery (#50163)

* Python interpreter discovery

* No longer blindly default to only `/usr/bin/python`
* `ansible_python_interpreter` defaults to `auto_legacy`, which will discover the platform Python interpreter on some platforms (but still favor `/usr/bin/python` if present for backward compatibility). Use `auto` to always use the discovered interpreter, append `_silent` to either value to suppress warnings.
* includes new doc utility method `get_versioned_doclink` to generate a major.minor versioned doclink against docs.ansible.com (or some other config-overridden URL)

* docs revisions for python interpreter discovery

(cherry picked from commit 5b53c0012ab7212304c28fdd24cb33fd8ff755c2)

* verify output on some distros, cleanup

test/integration/targets/interpreter_discovery_python/library/test_echo_module.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python
+# -*- coding: utf-8 -*-
+
+# (c) 2012, Michael DeHaan <michael.dehaan@gmail.com>
+# (c) 2016, Toshio Kuratomi <tkuratomi@ansible.com>
+# 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 sys
+from ansible.module_utils.basic import AnsibleModule
+
+
+def main():
+    result = dict(changed=False)
+
+    module = AnsibleModule(argument_spec=dict(
+        facts=dict(type=dict, default={})
+    ))
+
+    result['ansible_facts'] = module.params['facts']
+    result['running_python_interpreter'] = sys.executable
+
+    module.exit_json(**result)
+
+
+if __name__ == '__main__':
+    main()