Pass signal_data to heat client as dict rather than JSON

This patch removes JSON dump operations of signal_data in
heat-config and passes signal_data to heat client as dict.
This will fix the request body lost issue which causes
output of SoftwareDeployment resources unavailable when
using HEAT_SIGNAL.

Change-Id: I24848a875405a8b89cd629a064f774b7a80d25b8
Closes-Bug: #1355692
Merge "Pass signal_data to heat client as dict rather than JSON"