Move client creation functions to utils._clients

Potientally, other functions and scripts within os-cloud-config will
also want to query running OpenStack services, and rather than
duplicating code, let's move the functions that connect to nova,
ironic and keystone as a user into utils._clients.

Change-Id: Ic344de3d1550d4cb4d292882bf00f93cbb888931
Merge "Move client creation functions to utils._clients"