Make spliting system_search_domains more robust

The search line in /etc/resolv.conf could have
multiple spaces or tabs between domains.
split(' ') will give wrong results in some case,
use split() without argument instead.

e.g.
>>> 'domain.tld	cluster.tld '.split(' ')
['domain.tld\tcluster.tld', '']
>>> 'domain.tld cluster.tld '.split()
['domain.tld', 'cluster.tld']

roles/docker/tasks/set_facts_dns.yml

@@ -47,7 +47,7 @@
 
 - name: add system search domains to docker options
   set_fact:
-    docker_dns_search_domains: "{{ docker_dns_search_domains | union(system_search_domains.stdout.split(' ')|default([])) | unique }}"
+    docker_dns_search_domains: "{{ docker_dns_search_domains | union(system_search_domains.stdout.split()|default([])) | unique }}"
   when: system_search_domains.stdout != ""
 
 - name: check number of nameservers