scylla_util.py: fix "systemctl is-active" causes error
scylla_util.py: fix "systemctl is-active" causes error

On 48b6aec16a76f8cd60babf97cede99e7c0e50db1 we mistakenly allowed
check=True on systemd_unit.is_active(), it should be check=False.
We check unit's status by "systemctl is-active" output string,
it returns "active" or "inactive".
But systemctl command returns non-zero status when it returning
"inactive", so we are getting Exception here.
To fix this, we need new option "ignore_error=True" for out(),
and use it in systemd_unit.is_active().

Fixes #10455

Closes #10467

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