Invalid client version message unclear

The error message when using an invalid os-compute-api-version is
created by joining all the available versions without space.
This patch will add ', ' between versions.

Ex: nova --os-compute-api-version v3 list
ERROR: Invalid client version 'v3'. must be one of: 321.1
Now: nova --os-compute-api-version v3 list
ERROR: Invalid client version 'v3'. must be one of: 3, 2, 1.1

Change-Id: I825df89d38adc9e4bd3fb900cd0199f159c04a6b
Merge "Invalid client version message unclear"