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
client: add toprealm to krb5.conf (laptop)
Merge pull request #10 from opinsys/kerberos

client: add toprealm to krb5.conf (laptop)
Respect 79 characters perl line in puavo-update-client.
On puavo-update-client do use the basic auth with
ldapdn/ldappassword to authenticate laptops/wirelessaccesspoints.
This fixes doing configuration updates from "cloud-puavo".
In image updates (for laptops and such), use
"images.$(cat /etc/puavo/topdomain)" as a fallback image server
if a more immediate image server is not available.
Raise rate-limit for image update downloads by factor of ten.
puavo-update-client: Allow setting rate limit
by puavo-tags (tag format is imagedownload-rate-limit:[0-9]+k).
On ltspimage updates, read .imageservers[] from /etc/puavo/device.json
and take the first one if such exists.  This is used if dns
does not respond with anything sensible, but before
the "images.$(cat /etc/puavo/topdomain)" fallback.
Merge branch 'laptop_update_fixes'

Make laptops and wirelessaccesspoint updates work without
a bootserver.
ci: Workaround the environment issue on Trusty
All of puavo-ltsp-* depend on puavo-client
(the packages have scripts that call puavo-resolve-api-server,
which is in puavo-client package).
bootserver: update DNS records required by airprint after ddns init
bootserver: update airprint DNS records once per 15mins

This ensures printer permission/visibility changes in Puavo come into
effect with max 15min delay.

This kind of recurring and stupid update-loop is required until
bootservers can subscribe to Puavo event queues.
bootserver: install airprint DDNS update cron script
bootserver: fix trivial typo in airprint update cron script
client: Remove 50-default-display-manager as it was needed for ldm
and it messes up normal lightdm startup through upstart.
client: Import files from ltsp-lightdm to get rid of ltsp-lightdm
package.
client: Rename puavo-desktop-guest-session -> puavo-desktop-session