Fix empty ini files not including DEFAULT section

Empty files and subsequent modification by adding, removing,
setting and changing keys then outputting those files seems
to cause iniparse to ignore the whole DEFAULT section. To fix
this modify the write() function to correctly output the DEFAULT
section when it exists (but does not exist in the read data).

Fixes bug 1303968

Change-Id: I1b4133bc6f876e9e5e39045f46054db13bd4634f
Add pycadf + fix oslo.messaging requirement

Pycadf pulls in an older version of oslo.messaging, as
does most of the other packages. Since 1.3.0 is released
and we are building it allow the dependency resolver/adjuster
to correctly adjust those packages to use the specified
version instead of the one found in the requirements files.

Change-Id: I59b607839a8b65274a08a4903cafb98ab8900870