Define the matrix_dynamic_dns_domain_configurations variable in the role

Having it unset in the role itself (while referencign it) is a little strange.

Now people can look at the `roles/matrix-dynamic-dns/defaults/main.yml`
file and figure out everything that's necessary to run the role.

Related to #681 (Github Pull Request)

roles/matrix-dynamic-dns/defaults/main.yml

@@ -27,6 +27,17 @@ matrix_dynamic_dns_base_path: "{{ matrix_base_data_path }}/dynamic-dns"
 matrix_dynamic_dns_config_path: "{{ matrix_dynamic_dns_base_path }}/config"
 matrix_dynamic_dns_docker_src_files_path: "{{ matrix_dynamic_dns_base_path }}/docker-src"
 
+# Holds the configurations (the domains to update DNS for, the providers they use, etc.)
+#
+# Example:
+# matrix_dynamic_dns_domain_configurations:
+#   - provider: domains.google.com
+#     protocol: dyndn2
+#     username: XXXXXXXXXXXXXXXX
+#     password: XXXXXXXXXXXXXXXX
+#     domain: "{{ matrix_domain }}"
+matrix_dynamic_dns_domain_configurations: []
+
 # Config options
 matrix_dynamic_dns_additional_configuration_blocks: []
 matrix_dynamic_dns_use: "web"