dnsconfig: add support for 'action: member'.

This patch adds support for 'action: member' for ipadnsconfig plugin,
impacting management of DNS forwarders setting.

Use of 'state: absent' now requires 'action: member'. With 'state:
present', orwarders can be either defined through 'action: dnsconfig'
or added using 'action: member'.

Tests have been updated to reflec the new behavior.