SoftwareDeployments move signal reason into resource plugin

Move the determination of the signal reason string inside the resource
plugin so we don't need the resource-specific details handling in the
base-class.

Change-Id: I1eec87c2063a1958eba2ad32d43474c2ed435117
Related-Bug: #1340894
Merge "SoftwareDeployments move signal reason into resource plugin"