Simplify the individual form providers by always running reconfigure. This
also commits all UCI packages, so we can remove all tracking of UCI
sections from the wizard code.
Instead of returning a list of packages to commit, the providers just
save their settings directly. Explicit runs of upgrade scripts are
removed, as they are now run by reconfigure.
Before 7827f89, mandatory hostname field in config mode was
pre-filled with the default hostname.
This commit adds the config_mode.hostname.prefill option for
controlling the default value.
This setting allows to enforce manually setting a hostname.
In the initial configuration, the hostname field is now left empty; when
setting the hostname is not enforced, the default hostname is shown as the
field placeholder.
Fixes#1139
