Fix CentOS 6 installer issue (#6784)

In CentOS 6 default httpd configuration, the `LoadModule ssl_module ...` is handled in `conf.d/ssl.conf`. As the `VirtualHost` configuration files in `conf.d/` are loaded in alphabetical order, this means that all files that have `<IfModule mod_ssl.c>` and are loaded before `ssl.conf` are effectively ignored. This PR moves the `LoadModule ssl_module` to the main `httpd.conf` while leaving a conditional `LoadModule` directive in `ssl.conf`.

Features
 - Reads the module configuration from `ssl.conf` in case some modifications to paths have been made by the user.
 - Falls back to default paths if the directive doesn't exist.
 - Moves the `LoadModule` directive in `ssl.conf` inside `<IfModule !mod_ssl.c>` to avoid printing warning messages of duplicate module loads.
 - Adds `LoadModule ssl_module` inside of `<IfModule !mod_ssl.c>` to the top of the main `httpd.conf`.
 - Ensures that these modifications are not made multiple times.

Fixes: #6606

* Fix CentOS6 installer issue

* Changelog entry

* Address review comments

* Do not enable mod_ssl if multiple different values were found

* Add test comment

* Address rest of the review comments

* Address review comments

* Better ifmodule argument checking

* Test fixes

* Make linter happy

* Raise an exception when differing LoadModule ssl_module statements are found

* If IfModule !mod_ssl.c with LoadModule ssl_module already exists in Augeas path, do not create new LoadModule directive

* Do not use deprecated assertion functions

* Address review comments

* Kick tests

* Revert "Kick tests"

This reverts commit 967bb574c2d7d6175133931826cc2cdb4b997dda.

* Address review comments

* Add pydoc return value to create_ifmod

1 files changed, 9 insertions, 0 deletions

diff --git a/certbot-apache/certbot_apache/tests/testdata/centos6_apache/apache/httpd/conf.d/README b/certbot-apache/certbot_apache/tests/testdata/centos6_apache/apache/httpd/conf.d/README
new file mode 100644
index 000000000..c12e149f2
--- /dev/null
+++ b/certbot-apache/certbot_apache/tests/testdata/centos6_apache/apache/httpd/conf.d/README
@@ -0,0 +1,9 @@
+
+This directory holds Apache 2.0 module-specific configuration files;
+any files in this directory which have the ".conf" extension will be
+processed as Apache configuration files.
+
+Files are processed in alphabetical order, so if using configuration
+directives which depend on, say, mod_perl being loaded, ensure that
+these are placed in a filename later in the sort order than "perl.conf".
+