Commit 79a2342395 introduced an ability to
build 'in-house'/'not-built-in' projects with Kolla.
This commits extends this feature with an ability to use configure_user
macro for 'not-built-in' users.
The implementation is done the same way as for 'sources', i.e. engine
after registers new section structure as soon as it finds unknown
'<project>-user' section.
In addition, the documentation and CI are extended to cover
``--docker-dir`` option.
Change-Id: I690d9f3829083f2493bf286a1c45764b9699219b
cfa266a3a2