Selected time range is not considered when determining some variable options

When the possible options for the template variables Node and Interface are determined the current the current time range selection is not taken into account. This results in the default time filter (hard-wired in determined the backend) that spans the last 4 hours.

The current time range selection can easily be considered by supplying `$_from` and `$_to` as additional arguments to the corresponding datasource queries. This is already done for the (new) DSCP template variable.