RFE: Make prefix configurable via ENV

[atodorov]

As of now plugin_helpers.Backend receives a prefix string from the plugins but this is hard-coded.

In the plugin-demo repository I am using both tap and junit.xml plugins and I'd like for the TP prefix to be the same while for TR prefix to be different. This is a plausible scenario where you are running several test runs via different frameworks/plugins on a more complex product.

For example a mobile app with iOS and Android versions where the requirement is to have all TRs under the same TP, maybe for easier reporting of statistics.

Note: some of the changes will probably have to go into plugin_helper.py itself.