CLI improvements for reseller system setup

Summary:
Limit non-scalpel list commands to the tenant context
Add CLI tool to list SKUs
Add scalpel:sku:update command
Improve user:create for resellers
Update RESELLER.md

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4707