add describe section to each migration

[Ernest Micklei]

A migration list the commands to change Google infrastructure and optionally to revert it.
The tool gmig can show the status with respect to what migrations have been applied.
However, it does not tell you what the current status of your infrastructure is.
For that, you need to go to the Console or use the gcloud command to describe the resources.

This issue proposes to extend migrations files with a section that has the commands to do just that; describe the resources just created, changed or even deleted.

do:
- add a firewall rule

describe:
- show the created firewall rule

undo:
- delete a firewall rule

Alternative names for describe are view, list, show, inspect.
With this section, we can have a command such as

gmig view production/

which reports all the resources created by all migrations.