Make field validation to be conditional.

Introduce new field attribute 'enabled' - true by default, but if it
is false, then the field won't be validated, only coerced to Python
type. This actually solves the issue with hidden (and disabled) fields
which don't validate and prevent proceeding to next step.

Cherry-picked from release0.2, Ie57fa4731792ed1e44e9f61fa5cb6a16ddaf3109
Fixes: bug MRN-883

Change-Id: I4fc438a8949957800fcd0ac55bb30cd62744b1e0