Exists should return 'true' or 'false'

[devigned]

If an exists command is available for a resource (might not be needed if one were to use show), the only two valid returns should be 'true' or 'false'.

Currently, az group exists returns 'true' when exists and empty when it doesn't exist. By doing this, one can't say if [ $(az group exists -n group-that-doesnt-exit) == 'true' ] since that would cause a -bash: [: ==: unary operator expected to be thrown when null is returned.

[johanste]

Related to #1952.

[derekbekoe]

For az group exists, it looks like it behaves as expected now?

$ sudo docker run -it azuresdk/azure-cli-python:0.1.9
bash-4.3# az login -u <USER> -p <PASS>
bash-4.3# az group exists -n thisexists
true
bash-4.3# az group exists -n notthere
false

This command directly calls a method in the SDK. The SDK handles this as it checks for a 204 or 404 from the service - https://github.com/Azure/azure-sdk-for-python/blob/master/azure-mgmt-resource/azure/mgmt/resource/resources/operations/resource_groups_operations.py#L160.

Looking into the other exists commands now.

[derekbekoe]

The other exists commands are in storage module and they also have true/false results.

I suspect the az group exists issue was fixed by these two PRs.
#1854
#1913