Updated the Mongoid configuration options

Updated the Mongoid configuration so that the read and write modes can
be specified via the `forum_env`.

config/mongoid.yml

 common: &default_client
   options:
     write:
-      w: 1
+      w: <%= ENV['MONGOID_WRITE_MODE'] || 1 %>
     read:
-      mode: :primary
+      mode: :<%= ENV['MONGOID_READ_MODE'] || 'primary' %>
     max_retries: <%= ENV['MONGOID_MAX_RETRIES'] || 1 %>
     retry_interval: <%= ENV['MONGOID_RETRY_INTERVAL'] || 0 %>
     timeout: <%= ENV['MONGOID_TIMEOUT'] || 0.5 %>