add option for selective PTR generation on hosts.cgi

add option for selective PTR generation on hosts.cgi

In some cases, it might be useful to create an additional
host (i.e. for round robin loadbalancing) without assigning
another PTR to the IP address specified.

This patch introduces the ability to check or uncheck
PTR generation for each host individually.

Partially fixes #12030

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>