gluon-authorized-keys: add keys from site.conf

This package will run as invariant script after each upgrade and copy
all keys from site.conf's authorized_keys entry to
/etc/dropbear/authorized_keys.

Existing keys will be preserved.

The site.conf entry 'authorized_keys' is required (if this package is
selected) and must contain a list of strings, each representing a line
of the resulting file.