Introduce $api_version and @API_VERSION@

A bug fix minor release like 5.0.1 has no own API doc or CLIRR report
page. Add a new variable so that we can use those of the base release.

Change-Id: I1a5a60562427fe5d9fe68d080ece46f56227b180
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>