mongo: don't depend on apiserver/params package

This PR removes the dependency between the `mongo` package and the `apiserver/params` package.

The cost is we inline the four fields previously declared in the `params.StateServingInfo` api message.

The `mongo` package now depends on

github.com/juju/errgo
github.com/juju/errors
github.com/juju/juju/cert
github.com/juju/juju/juju/arch
github.com/juju/juju/network
github.com/juju/juju/replicaset
github.com/juju/juju/service/common
github.com/juju/juju/service/upstart
github.com/juju/juju/version
github.com/juju/loggo
github.com/juju/names
github.com/juju/utils
github.com/juju/utils/apt
github.com/juju/utils/proxy