Update "-l" to "--zone" in ibmcloud ks clusters

[mpetason]

Everything looks to be swapping over to zone. Using ibmcloud ks cluster -l should be changed over too, to match the new updates.

Right now you have to use -

$ ibmcloud ks clusters -l fra02

While vlans/other use -

$ ibmcloud ks vlans --zone fra02

Clusters only has the location option, not the zone option.

[steveclay]

@JohnStarich FYI request for feature.