See https://www.drupal.org/node/2400531
variable:boost_document_root
variable:boost_server_name_http_host
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variable:cache_lifetime
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variable:drupal_css_cache_files
variable:drupal_http_request_fails
variable:drupal_js_cache_files
variable:drupal_private_key
variable:emaillog_0
variable:emaillog_1
variable:emaillog_2
variable:emaillog_3
variable:emaillog_4
variable:emaillog_5
variable:emaillog_6
variable:emaillog_7
variable:entity_cache_tables_created
variable:environment
variable:environment_override
variable:error_level
variable:features_codecache
variable:features_ignored_orphans
variable:features_modules_changed
variable:features_semaphore
variable:install_task
variable:install_time
variable:javascript_parsed
variable:l10n_update_last_check
variable:l10n_update_translation_status
variable:mail_system
variable:maillog_devel
variable:maillog_log
variable:maillog_send
variable:maintenance_mode
variable:menu_masks
variable:module_filter_recent_modules
variable:page_cache_maximum_age
variable:picture_updated_to_file_entity_2
variable:preprocess_css
variable:preprocess_js
variable:security_review_last_run
variable:site_mail
variable:smtp_password
variable:smtp_username
variable:status
variable:update_last_check
variable:update_notify_emails
variable:xmlsitemap_base_url
variable:xmlsitemap_generated_last
variable:xmlsitemap_rebuild_needed
variable:xmlsitemap_regenerate_needed