Fix the ManagementNetValueSpecs param type
Fix the ManagementNetValueSpecs param type

The ManagementNetValueSpecs param type is currently set to string.
This change sets the param to the correct type of json, allowing the
network value specs to correctly parse.

Example Management Network value spec:

{'provider:physical_network': 'management', 'provider:network_type': 'flat'}

Change-Id: I5b12c7251690368d79a4d00725a9d6e0d5e75af8
Closes-Bug: #1573649

openstack/tripleo-heat-templates,trozet/opnfv-tht,openstack/tripleo-heat-templates,bcrochet/tripleo-heat-templates,trozet/opnfv-tht,bcrochet/tripleo-heat-templates,bcrochet/tripleo-heat-templates,trozet/opnfv-tht,dprince/tripleo-heat-templates,dprince/tripleo-heat-templates