Replace json with jsonutils from common code

We should use common code where possible.
This change is intended to remove direct usage of json
module and start use jsonutils from manilaclient.openstack.common

Also added lost dependency 'iso8601', that is used by jsonutils

Partially implements: blueprint use-common-code

Change-Id: I42ea803d761b2510c959dc1578a20ff155670283
Merge "Replace json with jsonutils from common code"