python-keystoneclient: Bump to 0.6.0

The 0.6.0 release is fully Python 3 compatible. Requiring at least this version
would help porting the clients that depend on the keystone client.

Change-Id: I3dc0d145f825d92a04f3a7197319f8147adc3913
Merge "python-keystoneclient: Bump to 0.6.0"