argparse is available from python2.7, but a lot of python2.7 compilations does
not provide it.

As users expect package to work by default, in case if python used to install
package does not contain argparse, add it to install_requires without requiring
any intervention, because default invocation shall work.

It will allow to minimize common dependency and require ZODB3 module.

This reverts commit d48e4455.

option one_disk and no_usb are no longer needed when using virtual
In case of virtual option remove kvm module activation

The binary "vmware-mount" has to be installed on the computer
option --virtual must be use with slapbuilder

On boot, slapos script check for Ipv6. If none is found it launch and use OpenVPN

Add possibility to choose  Self hosted image with option --one_disk