shipperctl: split "shipperctl admin clusters apply"

[hihilla]

This is a cherry-pick of #292

First of all, this does away with the "admin" namespace. Commands were getting too long for our taste.

Second, shipperctl admin clusters apply is now split into two,
supposed to be called in order, like so:

shipperctl clusters setup management
shipperctl clusters join -f clusters.yaml

The first only installs Shipper in the management cluster, and the
second joins application clusters to the management one. This allows us
to roll out updates to shipper without necessarily having to specify
application clusters to be joined.