Remove function get_software_config

This change removes the function get_software_config
and replaces it with direct calls to heatclient.

This allows the heat client plugin's ignore_not_found handling
to be used.

The SoftwareConfigMissing exception is also no longer required.

Change-Id: I61fbe9556d9988fffd4afddf3fad5d2fa211475a
Merge "Remove function get_software_config"