Add a metadata type validation when creating vm

If specifying non-dict data(string, etc.) as metadata, "create a vm"
API returns a HTTP500(Internal Server Error) response because it does
not validate the metadata type.

This patch adds the metadata type validation, and the API will return
a HTTP400(Bad Request) response. In addition, this patch removes log.WARNs
of bad incoming requests because they are just client-side problems not
server-side.

Closes-Bug: 1238344
Change-Id: I8f1d5debf74891e67b17f8128cc0eb0ab7f5c3f5
Merge "Add a metadata type validation when creating vm"