cmd/jujud: add help-tool subcommand

Now you can do:
    jujud help-tool: list all tools a la "help commands"
    jujud help-tool <tool>: show tool's help
Move help-tool command to cmd/juju

And add some tests.
cmd: Add --version to cmd.SuperCommand

There's no option to control the format;
the sub-command must be used for that.
Add comment to SuperCommand.SetFlags re non-global flags
cmd/juju: update failing unit test

Forgot to retest after moving from jujud to juju.
cmd.SuperCommand: revise docstrings
remove storage-container-name from config in preparation for generating one automatically
getEnvPrefix -> GetEnvPrefix and ise it in the storagecontext's getContainer()
Add container creation and deletion to environ
deleting container also removes its blobs
fix compile errors
Use GetAnonymousFileURL to get a file's URL.
Explicit test for SuperCommand/subcommand --version flag collision
Add check
Fix error case.
Fix object names.
wip
worker/upgrader: add UpgradeReadyError.ChangeAgentTools
gofmt
merge 353-upgrader-change-agent-tools