Add support for devfile 2.0 variables

[amisevsk]

Description

The devfile 2.0 spec includes global variables, which define key-value pairs, e.g.

variables:
  my-var: "some value"

These variables can be referenced in non-key string fields within the devfile using {{my-var}} to template different versions of a devfile by setting only the variable field.

We should add support for this in DWO.

See issue devfile/api#239 and PR devfile/api#352 for details.

[sleshchenko]

is implemented with #509

[sleshchenko]

duplicates #408