make yum group file option relative to repoPath

[dgoo2308]

allow to define

pathprefix=/mirror

[centos7-os]
name=centos 7 - x86_64
localpath=centos/7/os/x86_64
groupfile=comps.xml

and have the groupfile relative to the local path, as reposync places it in localpath

produces f.i. as per above yum file

createrepo --update --database --checkts --cachedir=/mirror/reposync/tmp/cache/repo/centos6 --workers=8 --profile --verbose --groupfile=/mirror/centos/6/os/x86_64/comps.xml /mirror/centos/7/os/x86_64