Improve communication PMM to Platform

Proposing the following improvements:

1. Using url.JoinPath instead of + operator here. Current implementation may lead to incorrect path building, e.g. it cannot handle leading and trailing slashes correctly or absent of them.
2. If response not in json format we cannot parse it and reporting parsing error but not the original one. So the original error cannot be found in the logs. The code 
3. Add more verbose debug-level logging, e.g. endpoint and body parameters.