support single collator config in `toml` (#253)

`v1` version support define a parachain with a single collator using
`[parachain.collator]` (note this is a map and not a sequence). But in
the `sdk` we drop support in favor of always use a vec<NodeConfig>. This
pr allow to use the old toml definition __only__ with the load_from_toml
method.

cc:  @AlexD10S