Fix "file not found" error.

Fix "file not found" error.

If the last stable release does not contain
a file, deploying a charm failed with a
"file not found" error. We encounter this 
problem, e.g., while uploading a new release:
the release is created in Launchpad, but the 
file takes a bit before being ready.
Now the relevant function tries previous
releases before failing.
Also fixed a functional test failing because
now staging does not require explicit
authentication (user and password are
provided by the config file).

https://code.launchpad.net/~frankban/charms/precise/juju-gui/bug-1102636-file-not-found/+merge/144290

(do not edit description out of merge proposal)

[frankban, 2013-01-22 11:46:28]

Please take a look.

[bac, 2013-01-22 12:09:35]

Francesco thanks for the quick turn-around on the branch.  Land as is.

I was confused that series.releases were returned latest first and had to turn
to the launchpadlib documentation to ensure that was correct.  Perhaps a comment
in the code where we use it might be helpful since our code depends on that and
the tests are constructed such that it is always true.  Maybe that's overkill
and people should just understand the library.  Do it only if you think that
makes sense.

[teknico, 2013-01-22 12:18:24]

Land as is.

Thanks, nice fix. +1 to bac's comment too.

[frankban, 2013-01-22 12:24:48]

Please take a look.

[frankban, 2013-01-22 12:26:08]

Hi Brad and Nicola,

thanks for your reviews, another test run and then I'll land this branch.

[frankban, 2013-01-22 12:49:53]

*** Submitted:

Fix "file not found" error.

If the last stable release does not contain
a file, deploying a charm failed with a
"file not found" error. We encounter this 
problem, e.g., while uploading a new release:
the release is created in Launchpad, but the 
file takes a bit before being ready.
Now the relevant function tries previous
releases before failing.
Also fixed a functional test failing because
now staging does not require explicit
authentication (user and password are
provided by the config file).

R=bac, teknico
CC=
https://codereview.appspot.com/7177049