Settle config load order of precedence

[bomoko]

There seems to be some weirdness with the order that config files are loaded.

If I pass a config file into lagoon-sync, I expect that config to be the one that's used before every other possibility.

We need to settle this order to something like

1 - if config is passed as argument, it's used
2 - any local (to the project) .lagoon.yml (if the lagoon-sync section appears)
2 - Anything set in the LAGOON_SYNC_DEFAULTS_PATH and LAGOON_SYNC_PATH env var
3 - eventually - fall back on default to the app itself.

The relevant code seems to be here https://github.com/uselagoon/lagoon-sync/blob/main/cmd/root.go#L60-L121

[timclifford]

I cansee this work has been partly started in #66

[timclifford]

This one got tackled in #90