scylla_setup: don't run node-exporter setup when it's not installed
scylla_setup: don't run node-exporter setup when it's not installed

We need to run package existance check before run setup of
node-exporter.

Fixes #8276

Closes #8278

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla