* plugin/kubernetes: remove bunch a string ops

This removes a bunch of appends to where not needed, makes dnsutil.Join
take variadic args which removes the need to wrap in a new string slice.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix calls to dnsutil.Join
Signed-off-by: Miek Gieben <miek@miek.nl>

* Revert these
Signed-off-by: Miek Gieben <miek@miek.nl>