Enable Specifying Instance Name

As per varnish's documentation and the default configuration file, although '$INSTANCE' is defined in the main configuration file, it is not used. Instance was always `(uname -n)`, regardless of the value specified for '$INSTANCE'.
Adding an -n switch to the varnish.erb template file to allow this to be specified.
(note that this switch is not used if the varnish::instance is set to 'default', to avoid any change in behaviour for people using the default option.)