Need namespace for mcloud-specific metadata

[Michi Henning]

Looking through the mcloud provider code, it the provider adds mcloud-specific metadata entries (which is fine). But these do not have any namespace. If we (in the framework) add a new well-known metadata item that happens to have the same name as a provider-specific one, we can end up with clashes (such as the framework version of the item having a different type).

The mcloud provider should prefix all its provider-specific metadata, that is, use "mcloud:suffix" and "mcloud:thumbnail_url", instead of "suffix" and "thumbnail_url".

[Michi Henning (michihenning)]

Also, I just noticed that some of the metadata keys don't use the symbolic constants from common.h. At least DESCRIPTION and CONTENT_TYPE could be used; there might be others.