subtests.docker_cli: Add run_dns subtest

This test executes container with various dns/dns-search settings
and verifies results. It tests booth correct and incorrect variants.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
docker_cli/run_dns: Update to use mustpass/mustfail

Signed-off-by: Chris Evich <cevich@redhat.com>
docker_cli/run_dns: Test 300-char hostname

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #424 from cevich/run_dns

Run dns