Overwrite config variables

Since newer version of capistrano sets the same variables, `_cset` are silently ignored and try to use capistrano's template, not capistrano-maintenance's one. (see https://github.com/capistrano/capistrano/pull/456 )

Assuming that `require 'capistrano/maintenance'` comes **before** setting those variables, this fix allows user to use capistrano-maintenance template by default.