maas provider's instance is not a Stringer

The MAAS provider's instance implementation does not define a String method. Thus, various log messages are not helpful in difficult situations.

<davecheney> 2013-09-09 06:05:37 DEBUG juju.provisioner provisioner_task.go:300 Stopping instances: [0xc2002a1be0 0xc2002a1c10 0xc2002a1c60 0xc2002a1c70 0xc2002a1c80 0xc2002a1cc0 0xc2002a1cd0 0xc2002a1b80 0xc2002a1ba0 0xc2002a1c20 0xc2002a1c50 0xc2002a1c90 0xc2002a1ca0 0xc2002a1cb0 0xc2002a1b60 0xc2002a1b70 0xc2002a1b90 0xc2002a1bb0 0xc2002a1bd0 0xc2002a1c30 0xc2002a1bc0 0xc2002a1bf0 0xc2002a1c40]
<davecheney> 2013-09-09 06:05:38 DEBUG juju.provider.maas environ.go:303 error releasing instance &{0xc20030e870 0xc2002beb40}
<davecheney> 2013-09-09 06:05:38 DEBUG juju.provider.maas environ.go:303 error releasing instance &{0xc20030e8a0 0xc2002beb40}
<davecheney> 2013-09-09 06:05:38 DEBUG juju.provider.maas environ.go:303 error releasing instance &{0xc20030e960 0xc2002beb40}