Add size per image (#480)

* Add backend part of adding size for images.
Creating an image set his instancesSize to the default value in config.
Creating instances will take the size specified in image.instancesSize.

* Add front-end part of adding a size per image

* Fix instances size of the different drivers,
Make test corresponding to the update of machine size per image.

* Delete configurations variable, replaced by instancesSize.

* Fixe name/indentation

* Update migration script

* Add informations for users.

* Readme update

* If machine have no flavor, it is recreated.

* Fix some bug when updating size.
Add column size on machines index page, and machine details pages.
Add the possibility to change default instances size for new images.
Update nanocloud api doc.

* Add tests to validate adding instances size per image.

* Add more visibility

* eslint, update timeout on machines service units tests.

* Update wording