Clean up db.sqla.Models.extra_keys interface

Previously we were checking that Class has
_extra_keys() method and if it has then extend columns with this list.

It will be much cleaner to have in ModelBase this method so we will be able
to avoid this check,  and as method should contains verb we change the name
also to _get_extra_keys().

It changes API so we should also make related in other projects.
But there is not so much work (e.g. in Nova there are only 2 occurences)

Change-Id: Ib9f3a165fae340a1c887769ce8f21a70c28458d0
Closes-Bug: #1119717
Provide useful defaults for config generator

Right now if you run generate_sample.sh in (for example) Nova,
you have to provide all three parameters for it to work right.
This change tweaks the defaults so that the script can be run
from the root of the project directory with no parameters.
This should apply to all of the projects, not just Nova.

Closes-Bug: 1204204

Change-Id: Iaa0a899dd4f34f44317f9574688e305365a604bf
Merge "Provide useful defaults for config generator"
Merge "Clean up db.sqla.Models.extra_keys interface"