`site_apt` aready ensures for installing packages after Exec[update_apt]
is run, so we don't need to duplicate this in `site_config::default.pp`.

Because this is the recommended way of depnending in the apt README.

`site_config::default.pp` takes care the all packages are
installed before `Exec['refresh_apt']`, so we don't need to
add it here for a single package.

The apt module now takes care of all the dependencies removed
from `site_apt`.
Also, the dependency to install the `lsb` package after
`refresh_apt` is unnesseccary because lsb facts won't work
anyway on the first run if `lsb` is not installed before, so
we can safely remove it.

- Resolves: #7842

Providing a custom sources.platform.apt.basic value worked
with the last commit, but without that the platform would fail.
So we provide a default value now in provider_base/common.json,
which can get overridden.

So we can use the experimental-0.8 repo instead of 0.8 i.e.

Use this to customize the main LEAP deb url:

    "sources": {
      "apt": {
        "leap": {
          "basic": "http://deb.leap.se/experimental-0.9"
        }
      }
    }

so we can easily use the experimental-0.(8|9) deb repos, which are
signed with this key