Code cleanup: use oslo's to_slug() instead of slugify()

The bash completion code is the sole user of the slugify() function
in utils, which is substantially similar to to_slug() provided in
strutils from oslo.

Change-Id: Ib4eb7e4c0fa0e9bc5c4a0856f6391911d8f8fd3b
Closes-bug: #1266118
Merge "Code cleanup: use oslo's to_slug() instead of slugify()"