common: Try lazy umount after 8 tries

clean_mount_points handle virtual filesystem, we can try lazy umount
after 8 tries.
common: mount /proc in bind

Let's use the same way on mounting /proc as the other /dev & /sys.
On one system, it failed and this commit solved the situation.
common: Using do_chroot

On some distro, we need to enforce the path to chroot, so let's use
do_chroot for that.
common: create /var/lib/edeploy if not existing

Since common_setup is called from base, we need to create the dir unless
it fails and break the complete setup.
packages: Adding get_openstack_mirror()

This helper is made to ease the definition of the openstack
repositories.

The default value can be overided by OPENSTACK_REPOSITORY variable