Enable 'validators' attribute for each field.

Previously they were available only at form-level. Field-level
validators could be used to validate against different regexps (with
YAQL 'test' function, which is also added in this commit) and show
different messages.

Closes-bug: #1250823
Change-Id: I7d4e1119f1ad8651abe0c7ff3f6fb56ff7d84694
Merge "Enable 'validators' attribute for each field."