Small improve of str2dict function

Change-Id: Ifa8974f2cbdac4fdc0cbd47d7ad9910a5bd31cb7
Leverage openstack.common.jsonutils

This patch removes our own home brewed json helper functions and leverages
the ones from openstack.common.jsonutils instead.

Change-Id: Idc5ee0fec56b0a0f337433a830647d4f421a8283
Closes-bug: 1365263
Merge "Leverage openstack.common.jsonutils"
Merge "Small improve of str2dict function"