Convert README from Markdown to ReStructured Text and use as longdesc (#22330)

* Convert README from Markdown to ReStructured Text and use as longdesc

Discussion in #13758 led to deciding to switch README to rst and having
setup.py consume it as the long_description.

* Fix long string in setup.py for pep8 compliance

* Open README.rst as read-only

* Update usages of root README.md to README.rst

Unsure about the file ./packaging/debian/docs as it (only) contains the
text README.md. I believe it's referencing ./packaging/debian/README.md
but maybe someone who knows debian packaging could review it?

* Pick up fixes that had been merged into README.md after the initial conversion to rst

setup.py

@@ -158,6 +158,17 @@ for extra_requirements_filename in os.listdir(extra_requirements_dir):
         with open(os.path.join(extra_requirements_dir, extra_requirements_filename)) as extra_requirements_file:
             extra_requirements[filename_match.group(1)] = extra_requirements_file.read().splitlines()
 
+try:
+    with open('README.rst', 'r') as readme_file:
+        longdesc = readme_file.read()
+except (IOError, OSError):
+    longdesc = ('Ansible is a radically simple IT automation system. It handles'
+                ' configuration-management, application deployment, cloud provisioning, ad-hoc'
+                ' task-execution, and multinode orchestration - including trivializing things like'
+                ' zero-downtime rolling updates with load balancers.\n'
+                '\n'
+                'Read the documentation and more at https://ansible.com/'
+                )
 
 setup(
     # Use the distutils SDist so that symlinks are not expanded
@@ -172,6 +183,7 @@ setup(
     name='ansible',
     version=__version__,
     description='Radically simple IT automation',
+    long_description=longdesc,
     author=__author__,
     author_email='info@ansible.com',
     url='https://ansible.com/',