add mongodb/reload_action attribute to enable config changes without a restart

This is my response to https://github.com/edelight/chef-mongodb/issues/55 and https://github.com/edelight/chef-mongodb/pull/36. It's a little misleadingly named since mongodb has no ability actually reload configs, but it is named that way for consistency with other cookbooks (and a hope that someday mongodb will have reloads).