Fix upload-tools for released versions; fix tests

Fix upload-tools for released versions; fix tests

(Cherry picked from juju-core to juju-core/1.16)

There was an issue with sync.Upload/SyncTools for
release versions. Upload sets Dev to true in SyncContext,
and calls SyncTools. SyncTools then sets Dev to false
because MinorVersion==-1. This caused SyncTools to fail,
since uploaded tools are *always* dev (version.Build++).

Also, several bootstrap and upgrade-juju tests have been
fixed for running in release versions (tested with
version.version=="1.16.0").

Finally, some test cleanup (don't connect commands
to stdio in tests):
- Added nullContext helper
- Make runCommand panic if ctx is nil
- Replace runCommand(nil...) with runCommand(nullContext()...)

Fixes #1237123

https://code.launchpad.net/~axwalk/juju-core/1.16/+merge/190085

(do not edit description out of merge proposal)

[axw, 2013-10-09 10:40:32]

Please take a look.

[fwereade, 2013-10-09 12:55:31]

LGTM