daemon: add a way to check for the version of augeas

Query augeas for its version when required, i.e. only once when using
the new augeas_is_version function.
appliance: daemon: import and use upstream shadow lens

Import the upstream lens for the shadow file, just with a different
identifier and not matching /etc/shadow by default. Instead, apply a
transformation to have it match /etc/shadow only if the version of
augeas is at least 1.2.1 [1].

[1] While the last upstream version is 1.2.0, all the development seems
to happen in master, so whatever the next version is going to be
numbered (e.g. 1.2.1 or 1.3.0), the check will be fine anyway.
sysprep: remove comment and notes about /etc/shadow lens

We are either using the upstream lens, or our copy of it, to handle
/etc/shadow, so now removing entries from it works.
customize: use augeas to change passwords

Make use of augeas to load and edit /etc/shadow, now that we have
(either from upstream or by ourselves) a lens handling it.