Saving Reana credentials

[JensKrumsieck]

Currently the execute remote feature uses --instance and --token to authenticate with reana.

maybe the credentials should be stored securely somewhere so that the user only needs to authenticate once (with an option to reauthenticate using another server, maybe key value store where key also contains instance url, to remember multiple servers??)

Maybe keyrring crate works for that

workflow.toml is to unsecure for token and i would prefer storing url and token in the same "place"

[JensKrumsieck]

as it was already changed to save into workflow.toml this issue was easy to implement :) Just changed storage