Merge lp://staging/~sinzui/juju-release-tools/cross-compile-win into lp://staging/juju-release-tools

This branch introduces two improvements to winbuildtest.py.

1. We get the stderr from the commands run by check_output.
2. The script always sets up cross-compilation for win amd64 and 386.

These two change allowed me to verify go is always ready for cross-compilation when we run it over ssh, and it confirms that win-deps are broken. I see this output when I ran a copy of this script on the win-slave

Command '('\\go\\bin\\go.exe', 'build')' returned non-zero exit status 1
COMMAND OUTPUT:
..\..\..\..\..\bitbucket.org\kardianos\service\service_windows.go:7:2: cannot find package "code.google.com/p/winsvc/eventlog" in any of:
 C:\Go\src\pkg\code.google.com\p\winsvc\eventlog (from $GOROOT)
 C:\Users\Administrator\ci\gogo\src\code.google.com\p\winsvc\eventlog (from $GOPATH)
..\..\..\..\..\bitbucket.org\kardianos\service\service_windows.go:8:2: cannot find package "code.google.com/p/winsvc/mgr" in any of:
 C:\Go\src\pkg\code.google.com\p\winsvc\mgr (from $GOROOT)
 C:\Users\Administrator\ci\gogo\src\code.google.com\p\winsvc\mgr (from $GOPATH)
..\..\..\..\..\bitbucket.org\kardianos\service\service_windows.go:9:2: cannot find package "code.google.com/p/winsvc/svc" in any of:
 C:\Go\src\pkg\code.google.com\p\winsvc\svc (from $GOROOT)
 C:\Users\Administrator\ci\gogo\src\code.google.com\p\winsvc\svc (from $GOPATH)

I feel vindicated that I can repeatedly setup cross-compilation. I don't think these deps are new, so maybe I was testing with an old tarball when I added win-agent buulding. agent building remains disabled.