Provide upgrade choices for stable and testing

[anxolerd]

It seems weird that by default stable version of vagga is installed in vagga-box but all updates are for testing version of vagga. The aim of this PR is to fix that and allow upgrades between stable versions of vagga in vagga_box.

This PR introduces the following changes:

• upgrade_vagga shell script can upgrade to either stable or testing version of vagga depending on command-line arguments passed
• upgrade_vagga by default upgrades vagga to the latest stable version. The motivation for this is that by definition testing version may contain unknown and unexpected bugs, which should not be present (by definition) in the stable version.
• vagga _box upgrade_vagga passes the first argument to the underlying shell script so the flags for upgrade_vagga can be used.

However still some help and clarification are needed. I have modified upgrade-vagga.sh but do not know how to rebuild image so my changes get into virtualbox. I suppose that some changes into code will also be required as the hash of the new image will be different from the hash of the current image.

[tailhook]

Looks good. But this PR will have to be postponed until I rebuild the virtualbox image (this isn't an automatic thing for many reasons).

[anxolerd]

Sure. By the way, how can I rebuild the image locally for my own machine?

[tailhook]

Use packer and image/vagga.json

[anxolerd]

Thanks