Let "create server" set custom metadata

Fixes #37

When creating a server, custom metadata "os" and "users" are
automatically derived from the image properties. This commit
allows users to overwrite them or add extra custom metadata
at server creation time.
Escape control characters in HTTP body, when json

Fixes #32

This commit ensures that control characters are escaped in all
HTTP response bodies, as long as they are parsed as json strings
Raw content is kept intact
Response header contents were already handled correctly