provider/manual: return hardware characteristics

StartInstance is updated to construct hardware
characteristics based on the node object returned
by MAAS in the result of "acquire".

We also fix StartInstance to use the architecture
of the acquired node to choose the appropriate
tools. We will acquire a node with an arbitrary
architecture unless specified, but we will now
guarantee correct tools.

Fixes https://bugs.launchpad.net/juju-core/+bug/1193998
Fixes https://bugs.launchpad.net/juju-core/+bug/1303853