luci-app-nut: use filesystem dependencies for configuration files

Don't use uci dependencies for nut related config files as the nut packages
ship with completely commented out uci files. Such files do not satisfy the
uci menu dependencies as those expect at least one section within the file.

To solve this issue, use a regular file dependency on /etc/config/nut_*
instead.

Supersedes: #5964
Signed-off-by: Jo-Philipp Wich <jo@mein.io>