qa/tasks/mgr: test_module_selftest set influx hostname to avoid warnings during plugi...

qa/tasks/mgr: test_module_selftest set influx hostname to avoid warnings during plugin test

This is a follow pr for pr#66376 and complete the set for influx server
start.

self-test will hit error MGR_INFLUX_NO_SERVER since we dont have
hostname configed, the following command will add a test hostname
so the error won't appear and fail the test.

Fixes: https://tracker.ceph.com/issues/72747
Signed-off-by: Nitzan Mordechai <nmordec@ibm.com>