Adding configurable S3 endpoint into director.yml.erb.erb 

Using Amazons S3 endpoint as default in the spec file.

An example in BOSH manifest might be:

```
  blobstore_endpoint: &blobstore_endpoint
    provider: s3
    s3_endpoint: blah.blob.com
    access_key_id: bosh
    secret_access_key: secretsquirrel!

  blobstore: 
    <<: *blobstore_endpoint
    bucket_name: bosh-source-blobs

  compiled_package_cache:
    provider: s3
    options:
      <<: *blobstore_endpoint
      bucket_name: bosh-compiled-package-cache
```

Personally I dont like have to represent these 2 differently, but the current director.yml.erb.erb kinda expects it that way. Maybe we should change this as well to make it more consisent so you can do this:

```
  blobstore_endpoint: &blobstore_endpoint
    provider: s3
    s3_endpoint: blah.blob.com
    access_key_id: bosh
    secret_access_key: secretsquirrel!

  blobstore: 
    <<: *blobstore_endpoint
    bucket_name: bosh-source-blobs

  compiled_package_cache:
    <<: *blobstore_endpoint
    bucket_name: bosh-compiled-package-cache
```