plainbox:unit: add purpose, steps and verification fields

This patch is first in series of patches that will split description field
into three distinctive parts of description: purpose, steps and verification.
checkbox-touch: use new fields

This patch makes checkbox-touch use new set of fields to describe jobs
that require human interaction/vericiation
plainbox:unit:job: add individual validators for new fields

This patch adds validators for purpose, steps and verification fields
checkbox-touch: add new fields references to IJobRunner WIP
plainbox:unit:job: description split cont'd
plainbox:unit:job: added tests for new fields
plainbox:docs: add info about new fields to job manpage

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
plainbox:abc: add new field properties to IJobDefinition

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
plainbox:docs add faq related to splitting of description field

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
plainbox:providers:stubbox: add representative for split fields

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge of lp:~kissiel/checkbox/description-field-split/ by tarmac [r=zkrynicki][bug=1375704][author=kissiel]"