stringify docker-squash version
stringify docker-squash version

Before:
(kolla) 11:46 (s) marcin@puchatek:kolla$ ./tools/build.py --squash
INFO:kolla.common.utils:Image squash is enabled and "docker-squash" version is b'1.0.8'

After:
(kolla) 11:46 (s) marcin@puchatek:kolla$ ./tools/build.py --squash
INFO:kolla.common.utils:Image squash is enabled and "docker-squash" version is 1.0.8

Change-Id: If1844434ff0b7c3b35d0a8e742c7e2fad0dc8758

openstack/kolla,openstack/kolla