metrics: nc: scaling nc should check `nc` is available

[grahamwhaley]

The metrics/scaling/k8s_scale_nc.sh test should check that nc is actually installed before it tries to use it. The check_cmds functionality is already there in the init() func, so just needs extending. Presently locally I see:

./k8s_scale_nc.sh: line 352: nc: command not found

[askervin]

Fixed in PR #212