install hook fails on xenial and later without retry

[Tom Haddon]

Currently the influxdb charm install fails without a retry because lib/influxdbcharmlib.py does 'import influxdb' at the top, which will fail the first time since the package is not installed. Then the reactive handler gets run, which apt installs the python3-influxdb package and imports it into the reative/influxdb namespace. But lib/influxdbcharmlib.py still does not have it imported. And then the handler checks its local 'has influx client' flag, calls a method in lib/inflxdbcharmlib.py, and the method fails because the module was never imported into that namespace.

The fix is to add 'influxdb' to wheelhouse.txt, pull out all the magic, and assume the import will work

[Stuart Bishop (stub)]

Or drop trusty support and remove all the import magic; it becomes a simple package install.