eliminate warnings of clippy

[developerworks]

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length comparison to zero
`#[warn(clippy::[len_zero](https://rust-lang.github.io/rust-clippy/master/index.html#len_zero))]` on by default
for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_zeroclippylen_zero
k8s.rs(33, 11): using `!is_empty` is clearer and more explicit: `!gateway_name.is_empty()`

called `map(..).flatten()` on an `Iterator`
`#[warn(clippy::[map_flatten](https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten))]` on by default
for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_flattenclippymap_flatten
k8s.rs(73, 24): try using `flat_map` instead: `.flat_map(|spec| spec.ports.iter())`