manual: SupportedArchitecture restricts add-machine to same arch as bootstrap-host

Some changes were made recently that restrict tools uploads to only those architectures that a provider/environment supports. The manual provider's implementation of supported architecture is too restrictive, including only the architecture of the bootstrap-host.

Since add-machine ssh:... can be used to provision a machine of any architecture that *Juju* supports, the manual provider should simply allow any architecture.

Note that there's also a question of manually provisioning into non-manual environments. As long as we require that machines be on the same internal network (this is a current limitation of manual provisioning w/non-manual environments), then provisioning to architectures not supported by the environment is a moot point.