LU-10973 lnet: dump cyaml to buffer

LU-10973 lnet: dump cyaml to buffer

Add a new API to dump cyaml to a buffer which gets returned to the
user. The user is then responsible for freeing the buffer.

This is a new API required by the LUTF in order to interface
with python more readily.

Test-Parameters: trivial
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I6068670f26b617d7a8366bd248dd80ca8cf15e5e
Reviewed-on: https://review.whamcloud.com/38082
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>