Support for cloud node removal

[jgleissner]

This adds support for removing cluster nodes in the public cloud. It uses salt-cloud to terminate the instance associated with the given minion, and uses a salt event processor to react on the salt-cloud destroyed event to remove the minion from the database.

This fixes https://bugzilla.suse.com/show_bug.cgi?id=1117152.

[jgleissner]

This requires https://github.com/kubic-project/caasp-container-manifests/pull/208 to work properly.

[MalloZup]

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