Ahaggar: add the output_local_prepare() to the buffer.* files

This function can be used to initialize a static output_buffer

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: update nodes-utils logic to use the new output_local_prepare()

output_local_prepare() is now declared in buffer.h, so just use it and
rename the global static output_buffer

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: improve the report() function

Pass the vars arguments to vasprintf()

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>