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. Remove slugify() and use to_slug() instead.

Change-Id: Iffab2f59504ee7ec6528860e06a5140ff22f332f
Closes-Bug: #1266127
Merge "Code cleanup: use oslo's to_slug() instead of slugify()"