Preliminary metadata signing

Preliminary metadata signing

Add an Encode funtion which can be used to sign
simplestreams metadata. Use the new Encode function
in tests which set up signed metadata and verify that
image and tools information can be read. This functionality
was until now not formally tested due to lack of ability to
sign arbitary metadata in juju.

https://code.launchpad.net/~wallyworld/juju-core/signed-simplestreams-metadata-tests/+merge/189755

(do not edit description out of merge proposal)

[wallyworld, 2013-10-08 04:38:11]

Please take a look.

[fwereade, 2013-10-09 08:15:45]

Confused about the empty public key and what will actually happen if we run this
live. Otherwise looking solid.

https://codereview.appspot.com/14524043/diff/1/environs/simplestreams/encode.go
File environs/simplestreams/encode.go (right):

https://codereview.appspot.com/14524043/diff/1/environs/simplestreams/encode....
environs/simplestreams/encode.go:45: err = plaintext.Close()
Should this have been deferred earlier?

https://codereview.appspot.com/14524043/diff/1/environs/tools/simplestreams.go
File environs/tools/simplestreams.go (right):

https://codereview.appspot.com/14524043/diff/1/environs/tools/simplestreams.g...
environs/tools/simplestreams.go:41: var simplestreamsToolsPublicKey =
`-----BEGIN PGP PUBLIC KEY BLOCK-----
What happens here in practice, live?

[fwereade, 2013-10-09 08:18:06]

On 2013/10/09 08:15:45, fwereade wrote:
> Confused about the empty public key and what will actually happen if we run
this
> live. Otherwise looking solid.
> 
>
https://codereview.appspot.com/14524043/diff/1/environs/simplestreams/encode.go
> File environs/simplestreams/encode.go (right):
> 
>
https://codereview.appspot.com/14524043/diff/1/environs/simplestreams/encode....
> environs/simplestreams/encode.go:45: err = plaintext.Close()
> Should this have been deferred earlier?
> 
> https://codereview.appspot.com/14524043/diff/1/environs/tools/simplestreams.go
> File environs/tools/simplestreams.go (right):
> 
>
https://codereview.appspot.com/14524043/diff/1/environs/tools/simplestreams.g...
> environs/tools/simplestreams.go:41: var simplestreamsToolsPublicKey =
> `-----BEGIN PGP PUBLIC KEY BLOCK-----
> What happens here in practice, live?

As discussed live, this is moot because tools data isn't signed yet anyway. LGTM
for trunk and 1.16