Fix mail_virtual_domains config reading in opendkim

[nfedyashev]

without this change it fails on ansible(2.2.0.0) with the following message:

TASK [mailserver : Generate OpenDKIM keys] *************************************
fatal: [sovereign.host]: FAILED! => {"failed": true, "msg": "the field 'args' has an invalid value, which appears to include a variable that is undefined. The error was: 'unicode object' has no attribute 'name'\n\nThe error appears to have been in '/Users/nfedyashev/Projects/sovereign/roles/mailserver/tasks/opendkim.yml': line 19, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Generate OpenDKIM keys\n  ^ here\n"}

[nfedyashev]

I'm not an expert in ansible but that update allowed me to continue running this playbook

[jplock]

There are a few other places in the roles where we need to fix this too. Thanks!