Include the DRBD upgrade tool in the dist archive

This patch adds the DRBD upgrade tool in the archive and in the
installed tools dir, and its associated README in the doc_DATA target.

Reviewed-by: ultrotter

tools/Makefile.am

 toolsdir = $(pkglibdir)/tools
-dist_tools_SCRIPTS = lvmstrap burnin cfgshell cfgupgrade
+dist_tools_SCRIPTS = lvmstrap burnin cfgshell cfgupgrade drbd8-upgrade
+dist_doc_DATA = README.drbd8-upgrade