Add safety belt to destroy-environment

Add safety belt to destroy-environment

Destroy-environment is dangerous and we should prevent people from shooting themselves in the foot if at all possible.  This change removes the -y flag from destroy-environment and changes the "are you sure?" prompt to force the user to type "destroy <environment_name>".  The flag is too easy to pass without ththinking, and the y at the prompt is similarly too easy to type in without the user actually thinking about what they're doing.  Forcing them to type the environment name will also ensure they don't accidentally destroy the wrong environment by accident.

https://code.launchpad.net/~natefinch/juju-core/022-destroyer/+merge/191501

(do not edit description out of merge proposal)

[natefinch, 2013-10-16 20:50:46]

Please take a look.

[rog, 2013-10-17 07:45:25]

On 2013/10/16 20:50:46, nate.finch wrote:
> Please take a look.

Please no!

I want to be able to use juju easily in scripts. This is
Unix and our target audience are developers and admins. Please
allow us to do dangerous things on the command line.

[natefinch, 2013-10-17 17:46:09]

Please take a look.

[natefinch, 2013-10-17 18:54:12]

I updated the code so that the -y flag is back, but you must always specify the
environment... it'll never take the current or default environment. 

The format of the command is now:

juju destroy-environment <envname> [-y|--yes]

Note, I chose not to use the normal -e environment flag because a flag implies
it's optional (which it's not).

[thumper, 2013-10-31 22:06:09]

LGTM

I think this is an overall improvement to the UI.  Still scriptable IMO.

https://codereview.appspot.com/14502070/diff/5001/cmd/juju/destroyenvironment.go
File cmd/juju/destroyenvironment.go (right):

https://codereview.appspot.com/14502070/diff/5001/cmd/juju/destroyenvironment...
cmd/juju/destroyenvironment.go:54: scanner := bufio.NewScanner(ctx.Stdin)
Just curious, but what benefit do we get over using bufio.NewScanner over
fmt.Fscanln?

[natefinch, 2013-11-04 21:33:55]

On 2013/10/31 22:06:09, thumper wrote:
> LGTM
> 
> I think this is an overall improvement to the UI.  Still scriptable IMO.
> 
>
https://codereview.appspot.com/14502070/diff/5001/cmd/juju/destroyenvironment.go
> File cmd/juju/destroyenvironment.go (right):
> 
>
https://codereview.appspot.com/14502070/diff/5001/cmd/juju/destroyenvironment...
> cmd/juju/destroyenvironment.go:54: scanner := bufio.NewScanner(ctx.Stdin)
> Just curious, but what benefit do we get over using bufio.NewScanner over
> fmt.Fscanln?

 fmt.Fscanln actually fails to return the entire string, and returns an error
about no EOL.  We were ignoring it, but it bit me when I was twiddling with the
code and tried to get it to read the full line... so I figured it was better to
make the code a good example rather than a bad example, even if functionally it
behaved the same in this particular case.