support pre-hook in docs-build.sh

support pre-hook in docs-build.sh

Some projects need special pre steps before executing docs-build.sh .
For instance, opnfvdocs needs to download other repos which will be
composited. Also, apidoc creation is needed during docs build in
yardstick.

In order to support such cases, this patch introduces pre-hook in
docs-build.sh that provides much flexibility to projects and
simplifies job definition in releng.

Change-Id: If9f26c490671ec0788dc48dd4fccb4513d0ca777
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>