Fpb, fix repo building for centos

Create repo generated wrong paths to packets when
we specified Packages directory, it thought that
packages were located in root directory of the
repository, it's not required to specify this
directory explicitly, and in this case createrepo
generates right paths to the packages.

Change-Id: I8742c3c37637a66b2f335bad32dfecea4f3ba9aa
Implements: blueprint cinder-neutron-plugins-in-fuel