Allow multiple targets in dox.yml

Try to emulate tox -e type of target/environment.

This works natively in tox.ini as long we have commands to run there
(doesn't support inheritance).

For dox.yml the format has been improved, the format looks now like this :

target1:
    images:
    commands:
    add:
    prep:

target2:
    images:
    commands:
    add:
    prep:

we still support old format with single target. 

Follow up patches in this series improve this implementation.

Change-Id: I2a30aaa63fd4e47fd76f29ed5e5cb025ca326016
Merge "Allow multiple targets in dox.yml"