Make cloud integration optional

[johnsca]

When deploying on a cloud that supports an integrator charm (aws, gcp, openstack, etc), the integrator charm is automatically added with no way of deselecting it. There should be a Yes / No prompt for installing it, defaulting to Yes.

Ideally, this would not be shown if a cloud that doesn't support an integrator is chosen, but this depends on conjure-up/conjure-up#1341