Add support for --non-interactive

[cheungpat]

Planning to add --non-interactive / SKYCLI_NON_INTERACTIVE, such that skycli will fail if a required parameter is not already specified via command line options and/or environment variable. One such example is skycli login, which ask for email and password if they are not provided. Enabling this option will make skycli login fail if email and password are not provided.