create empty known_hosts

adressing warings from ansible:
previous known host file not found

playbooks/hosts/copr-be-dev.cloud.fedoraproject.org.yml

@@ -61,6 +61,9 @@
   - name: setup copr user ssh config file
     action: copy src=$files/copr/ssh_config dest=/home/copr/.ssh/config  owner=copr group=copr mode=600
 
+  - name: create empty known_hosts
+    action: copy src=/dev/null dest=/home/copr/.ssh/known_hosts owner=copr group=copr mode=600
+
   - name: replace bashrc for copr user
     action: copy src=$files/copr/copr_bashrc dest=/home/copr/.bashrc owner=copr group=copr mode=600