Provide a default value for Ironic cleaning_network configuration

Provide a default value for Ironic cleaning_network configuration

Ironic will soon refuse to start when at least some value is not provided.
Unfortunately, we do not create any overcloud[*] networks during deployment.
Fortunately, Ironic does not validate this value until actual cleaning. So,
this change sets it to "provisioning", which is what people often use.

An update will follow to the documentation to recommend this name:
http://tripleo.org/advanced_deployment/baremetal_overcloud.html#configuring-cleaning

A new parameter is created for this value, with a reminded to change it to
an actual UUID later on. While a pre-defined name will work in a simplest case,
in a real multi-tenant deployment a network name conflict is possible.
Using a UUID is safer in this regard.

[*] networks created in overcloud neutron

Change-Id: I1b7dc2ff70d3b76f19a183a60e88cf72f6d2a318
Closes-Bug: #1661082