Tezt/Cloud: Add main dns entry

What

• Allow to register a dns entry for one or multiple gcp zones

Also small refactor to better accomodate with DNS terminology

Why

To provide user friendly dns name for bootstrap nodes

# How registering a new dns entry specified by --dns-domain

How to test

dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --log-file /tmp/log --proxy --dns-zone "tezt-cloud" --dns-zone "dal-nomadic-labs-com" --dns-domain "my-test"

Should register

my-test.dal.nomadic-labs.com.  A     300    34.34.155.95

and 

my-test.nl-dal.saroupille.com.        A     300    34.34.155.95