Do not allow relative path for installroot
Do not allow relative path for installroot

With relative path installroot doesn't work correctly. This also
restores behavior of dnf-2.7.5

Closes: #709
Approved by: jrohel

rpm-software-management/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,hughsie/libhif,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libhif