[March customer interview] Consistent idempotency of commands

While many Azure CLI commands are idempotent, I regularly run into situations where commands aren't. This results in having to write extra logic in scripts dealing with the lack of idempotency. It would be quite valuable when idempotency is positioned more strongly and clearly in [doc/command_guidelines.md](https://github.com/Azure/azure-cli/blob/master/doc/command_guidelines.md) and validating idempotency would be part of the test suite.