mail: 'my' location is available as mail location

move the reserve domains check to app location validation code

src/test/dns-test.js

@@ -18,10 +18,6 @@ describe('DNS', function () {
     after(cleanup);
 
     describe('validateHostname', function () {
-        it('does not allow admin subdomain', function () {
-            expect(dns.validateHostname('my', domain)).to.be.an(Error);
-        });
-
         it('cannot have >63 length subdomains', function () {
             const s = Array(64).fill('s').join('');
             expect(dns.validateHostname(s, domain)).to.be.an(Error);