[devstack] Copy bash_completion script during zun installation
[devstack] Copy bash_completion script during zun installation

Copy zun.bash_completion script to /etc/bash_completion.d
so that users/developers can take advantage of auto completion
of zun commands.

Closes-Bug: #1657004

Change-Id: Icb47fe4864176d4ca3699b0616f2c222eba22ae3

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun