Added support for verifying node DNS A records in puppetmaster::validation

This helps detection and debugging of certain exported (firewall) resource
problems. Note that this functionality is disabled by default and has to enabled
using the $check_a_records parameter. This change also adds the option to
disable ERB template check using the $check_erb parameter.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>