common: mounting virtual fs before lsb_release

In some case, the lsb_release fails as no /dev/null exists.
Let's reorder this part and mount the virtual fs as soon as possible.

This patch was after seeing this behavior :

    ./openstack-common.install /var/lib/debootstrap/install/RHEL7-F.1.0.0/base /var/lib/debootstrap/install/RHEL7-F.1.0.0/openstack-common RHEL7-F.1.0.0
        Copying tree /var/lib/debootstrap/install/RHEL7-F.1.0.0/base/ to /var/lib/debootstrap/install/RHEL7-F.1.0.0/openstack-common/done
        Running edeploy on top of git 320f7eebf405960b40ac39422a647362e02ad807
        /usr/bin/lsb_release: line 150: /dev/null: No such file or directory