Add validate_domain_name function

[frapex]

Add validate_domain_name function.

[tphoney]

I am struggling to see how this is different to https://github.com/puppetlabs/puppetlabs-stdlib/blob/master/lib/puppet/parser/functions/is_domain_name.rb can you explain ?

[frapex]

For almost every is_something function there is also a validate_something function (is_absolute_path and validate_absolute_path, is_email_address and validate_ip_address, et cetera)

There is no validate function available for the is_domain_name function. This commit adds the missing validate_domain_name function.

[frapex]

@tphoney can you please look at this pull request and merge it? It has been almost four months since the pull request was first created and almost three weeks since I added additional info and rebased.

[tphoney]

Thanks for this @frapex apologies for the delay, i was stuck on other things.

[frapex]

@tphoney thanks!