Add a new mixin for for cloudstack metadata.

The cloudstack api is sort of similar to the ec2 metadata api,
therefore we base our mixin on that and override only what is
needed:

The endpoint does not have a fixed ip to query, rather you have to
first find your dhcp server (a.k.a virtual router), then always
query the api endpoint using 'latest' as path.

The other parts should look enough like each other.
Add cloudstack plugin.

Add support for querying the cloudstack meta-data server.
Expose cloudstack information in cloud mash.

This adds generic cloud information to the cloud mash as well as
a few cloudstack specific values.