Show warning when auth_version >= 2 and keystoneclient is missing

Swiftclient hangs forever without any reasonable hint if someone
uses a recent version of swiftclient together with auth version
2 or later. The reason for this is the sys.exit() call in a thread
- the main thread never exists, and swift just waits forever.

Raising a ClientException instead of calling sys.exit() fixes this
and returns the desired information back to the user.

Change-Id: I1fdff85da2dd4b883d337d6098b1695e448d6f2b
Merge "Show warning when auth_version >= 2 and keystoneclient is missing"