Extend Postgres GUCs validator (#2671)

* Use YAML files to validate Postgres GUCs through Patroni.

Patroni used to have a static list of Postgres GUCs validators in
`patroni.postgresql.validator`.

One problem with that approach, for example, is that it would not
allow GUCs from custom Postgres builds to be validated/accepted.

The idea that we had to work around that issue was to move the
validators from the source code to an external and extendable source.
With that Patroni will start reading the current validators from that
external source plus whatever custom validators are found.

From this commit onwards Patroni will read and parse all YAML files
that are found under the `patroni/postgresql/available_parameters`
directory to build its Postgres GUCs validation rules.

All the details about how this work can be found in the docstring
of the introduced function `_load_postgres_gucs_validators`.

setup.py

@@ -157,7 +157,10 @@ def setup_package(version):
         long_description=read('README.rst'),
         classifiers=CLASSIFIERS,
         packages=find_packages(exclude=['tests', 'tests.*']),
-        package_data={MAIN_PACKAGE: ["*.json"]},
+        package_data={MAIN_PACKAGE: [
+            "postgresql/available_parameters/*.yml",
+            "postgresql/available_parameters/*.yaml",
+        ]},
         install_requires=install_requires,
         extras_require=EXTRAS_REQUIRE,
         cmdclass=cmdclass,