Decide what to do if the weave-cloud secret already exists

[aaron7]

Currently the bootstrap exits if the weave-cloud secret already exists.

Some options:

• give the option of overriding the secret (or overrride by default)
• let the user remove the secret themselves
• skip creating the secret and reapply the agent manifest

[aaron7]

Fixed by #31 - asking the user whether they would like to override the secret.