Added option to instruct mkcloud to configure the vlan manually

I ran into a case where ifup refused to bring up the VLAN, but if it
is configured using vconfig and ifconfig, everything works as expected.

This change doesn't change the default behaviour, to use the manual
configuration `vlan_manual_setup` environment variable has to be set.

It seems the problem is due that `cloudbr.300` (defined at `/etc/sysconfig/network/ifcfg-cloudbr.300`) depends on `cloudbr` which is configured by libvirtd, so ifup can't satisfy the dependencies for `cloudbr.300` and fails.