client: read device tags from device.json instead of lts conf

This change makes school-wide device tags to have an effect as
well. Previously tags were read only from lts conf, which was written by
puavo-client, which did not considered only device-specific tags when
writing the conf.

Both tag handler scripts were moved to a bit later stage because they
require /etc/puavo/device.json which is written by
90-puavo-setup-puavo-conf.
client: fix device tag matching to be exact, not fuzzy