Bug: Directly deploy leap-archive keyrings

The leap-archive keyring expired March 8th 2018.
We updated it, and published updated installation
docs at https://bitmask.net/en/install/linux.

For jessie, we dont install the leap-archive-keyring
package anymore but directly deploy the keys to
apt's trusted keystore.

- Fixes: leap/bitmask-dev#9279