Only ask for secret replacement if it's a different one than the one already there

[dlespiau]

The way I read the code:https://github.com/weaveworks/launcher/blob/12815eb19bf4c8d62072e77b7d26c87aae8e2df4/bootstrap/main.go#L95 we're always asking if the user wants to replace the secret (with a somewhat confusing message). We could do better by not asking if the stored secret and the incoming one are are the same.

[rade]

We could just not prompt ever. That would certainly be an improvement on the status quo. We could also inform the user, without prompting.

[dlespiau]

Remove or narrow down the boostrap prompt about the weave-cloud secret