nm dns: Only purge global DNS when required

When applying the same DNS configuration twice via nmstate,
NetworkManager always emit `dns-change` dispatch event.

This is caused by nmstate always purge global DNS configuration even
it will set global DNS afterwards.

To fix the problem, we only purge global DNS when iface-dns is used.

Creating test case for checking `dns-change` event is testing
NetworkManager instead of nmstate, hence no integration test case
included. Manually tested.

Resolves: https://redhat.atlassian.net/browse/RHEL-62106

Signed-off-by: Gris Ge <fge@redhat.com>