Volume v2 service clients pass api_version in __init__() now, but
Iddd8306723c1ff33105f513c1993a0497a949c29 will move the passed
api_version to class values for avoiding a redundant __init__()
definition.
For doing this, we need to move api_version to class value to avoid
initializing it in RestClient __init__().
Change-Id: Ic86739dde83dcac8f68e53599967de53694f692f