The function pretty_log fails with zero length in the test set_custom_hostname

[Egor Kotko]

The function pretty_log fails with zero length in the test set_custom_hostname

https://product-ci.infra.mirantis.net/view/8.0_swarm/job/8.0.system_test.ubuntu.custom_hostname/76/testReport/%28root%29/set_custom_hostname/set_custom_hostname/

Scenario:
            1. Revert the snapshot
            2. Create a cluster
            3. Add 3 nodes with controller role
            4. Add 1 node with compute role
            5. Set custom hostnames for all cluster nodes
            6. Deploy the cluster
            7. Verify network configuration on controller
            8. Run OSTF
            9. Verify that there are no dead services compute services
            10. Verify the new hostnames are applied on the nodes

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/258930

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/258930
Committed: https://git.openstack.org/cgit/openstack/fuel-qa/commit/?id=3acd8e818b75def0bb998d422f7404ee0dad366b
Submitter: Jenkins
Branch: master

commit 3acd8e818b75def0bb998d422f7404ee0dad366b
Author: Dennis Dmitriev <email address hidden>
Date: Thu Dec 17 11:58:07 2015 +0200

    Fix pretty log if input data is empty

    Change-Id: I9fa013ead48462d80e89b2149e6a4d7ff91cb8aa
    Closes-Bug:#1527190