Implement unit tests for kamaki.clients.astakos

Old unitests were testing the old AstakosClient class, which
is not been used anymore. The new package inctroduces wrapper
classes for the synnefo.AstakosClient class. The later is not
tested in this patch, but the wrapper classes and kamaki-specific
extentions are.
Fix minor errors in CycladesClient unit test
Update kamaki.clients.pithos unit tests
Fix minor errors in kamaki.cli unit tests
Remove test of deprec. method "_raise_for_status"