add search domain for split-dns setups

[akostadinov]

This particularly helps with DNS routers that filter private IP ranges
from responces of public DNS servers.

It is also possible to be done on server side with the option:

push "dhcp-option DOMAIN-SEARCH <SEARCH_DOMAIN>"

Note that this is an openvpn client 2.x option. For 3.x not sure this is supported yet. see https://community.openvpn.net/openvpn/ticket/1209

But in Fedora 36 I see OpenVPN 2.5 being used.

The option is described in man openvpn.

I COULD NOT TEST THIS AS I'M USING Network Manager!!!

[3scale-robot]

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[3scale-robot]

Welcome @akostadinov! It looks like this is your first PR to 3scale-ops/aws-cvpn-pki-manager 🎉