Fix cronjob and config bugs

Fixes the following bugs:

- Main cron job had invalid name that caused it to be ignored silently by run-parts.

- hookenv.config was not being saved, so 'run' was always considered changed, and thus fastpoll job was always re-added, even if only 'frequency' was changed

- changing 'frequency' alone should not re-install the poll job

- changing 'frequency' while 'run' is false should not install the job.