Fix kolla-bootstrap-servers with docker_storage_driver set

If docker_storage_driver is set in globals.yml, then kolla-ansible
bootstrap-servers is run, it fails like so:

The conditional check 'docker_storage_driver' failed. The error was: error
while evaluating conditional (docker_storage_driver): 'devicemapper' is
undefined

Ansible does not like evaluating strings as conditionals. This change switches
to using the length filter.

Change-Id: Ib95cbdac2a659e7a0d5f113fe48046de3a39dcda
Closes-Bug: #1849691