Fixed kgoss kubectl cp issue

[gaganyaan2]

Fixed https://github.com/aelsabbahy/goss/issues/770 issue

kubectl cp <file-spec-src> <file-spec-dest> file-spec-dest should be filename with path(optional) but It cannot be directory.

Reference for kubectl cp

• https://kubernetes.io/docs/reference/generated/kubectl/kubectl-commands#cp
• https://stackoverflow.com/questions/52407277/how-to-copy-files-from-kubernetes-pods-to-local-system

[aelsabbahy]

Tagging @joshgav , if this looks good to you, I'll merge.

[joshgav]

LGTM, the get_pod_file function only copies one file at a time anyway, and this makes that more resilient.

Thanks @koolwithk.

[aelsabbahy]

Thank you both for your contributions, greatly appreciate it!