inventory

This patch modifies the image building script by adding:

- An usage message.
- An option "-I" to NOT install IPA to the generated container.
- An opiton "-c NAME" to both set the name and use an existing container
  to ONLY install IPA.
- Rename "scenario" to "DISTRO" as "scenario" should be used for the
  container scenario usage, rather than the distro (I'll change the name
  also in the Azure scripts)
- Use 'log' (from shlog) to print messages.