pkg commands: creates a function newXCmd() in charge of creating a new command

[itscaro]

In some test cases, we need to reset flags & args of the command in test. In the actual used technique, creating the command in global scope and initializes its flags & args in init(), we cannot re-create the command to test it from state-zero.

If there is a testing technique for this case, please enlighten me :-)

Attempt to address this issue: https://github.com/openfaas/faas-cli/compare/master...itscaro:refactor-cmd

[itscaro]

Derek add label: proposal