Ensure creators' constructors cannot raise exceptions.

Ensure creators' constructors cannot raise exceptions.

While creating unit tests for vPing that is using SNAPS, exceptions can
be raised when retrieving their respective OpenStack clients with
incorrect credentials. Moved client retrieval to the create() method
which one should expect exceptions to be raised.

JIRA: SNAPS-81

Change-Id: Id8e4279f60b97704b25a89c6312dcf2ebdc3f459
Signed-off-by: spisarski <s.pisarski@cablelabs.com>