Make blobstore endpoints configurable

Hi,

I added a new option `endpoint` to blobstore settings to upload/download files to S3-compatible blobstores. Users can use their own (on-promise) blobstores with this option.

I found that `buildpack` blobstore shared its settings with `droplet` and I thought this was a bug, because, in cf-release, the template file of ccng has a `buildpack:` field. I fixed this problem too.

(I propose replacing `Fog::Storage` with the blobstore client in the feature. It's more flexible and I guess some users want to use other blobstores such as Swift and Atmos.)