Install doc files if they found for OpenStack clients

Instead of creating empty documentation files for client that do not
have them we now package them only if they exists. We also package
man page if it is installed to buildroot.

Closes-bug: #1284065
Change-Id: Ibe3bfd9793d1dfcdeb5c59d96725a9896595ad98
Merge "Install doc files if they found for OpenStack clients"