Dont fail when adding a duplicated domain

Currently we exit when trying to add a duplicated domain, even if we give a list
of domain names. This behaviour is not as it suppose to work.

**Expected behaviour**
When adding a domain, and the domain already exists, just skip that domain, and
continue with the world. This is no problem since we are already in the state we
want to be in.

**New behaviour**
When you try to add a domain that is already added, we notify the user of this
by showing a log message: `Skipping: example.com already added to example app`,
and then we continue with the list of domain names.

Fixes #2165

tests/unit/20_domains.bats

@@ -49,7 +49,7 @@ teardown() {
   run dokku domains:add $TEST_APP test.app.dokku.me
   echo "output: "$output
   echo "status: "$status
-  assert_failure
+  assert_success
 }
 
 @test "(domains) domains:remove" {