analytics-host should allow specifying a livereport class by path

Currently you must `pip install` a python package with a specific entrypoint to run a LiveReport in `analytics-host` using the `-t` argument. The `-t` argument should instead support "path-like" report template names and instead manually load the python class from a `.py` file:

Example code for explicitly importing a `.py` file and loading a class that derives from a known base class is:
https://github.com/iotile/coretools/blob/master/iotilecore/iotile/core/hw/hwmanager.py#L765

A good potential way to know if the user intended to specify a file manually is:
- the template name ends in `.py`