By default the service resource that manages the BIND service will
restart the service when notified by configuration changes. This can be
disruptive.

This change allows the user to set a custom restart command via
`dns::service_restart_command` for the service resource, such as
`/usr/sbin/service bind9 reload` or `/bin/systemctl reload bind9.service`
or even `/usr/sbin/rndc reload`, to avoid restarting the whole named
process. By default no restart command is set.

Make use of the `validate_cmd` parameter in concat to run
named-checkconf on the target file. This way the validity of to-be-updated
configuration files named.conf or zones.conf is checked before actually
writing the new configuration and restarting the named service. This
prevents named from loading invalid configuration settings that would
result in named failing to start.

The file named.conf.options itself cannot be checked with
named-checkconf because its content is only valid inside the
"options { };" directive.

When zonetype => forward is set for a dns::zone resource then ignore any
forbidden options that might be set as parameters when rendering the
named.zone.erb template. Such forbidden options are "file", "masters",
"allow-transfer", "allow-query", "notify" and "also_notify".

This prevents the module from generating an invalid named configuration
(in zones.conf) that would result in named failing to (re)start.

Fixes GH-141