Build the VM and after add GNS3

[julien-duponchelle]

For speed up the build process and avoid problems we need to build a VM release in two steps.

• Step 1 build the base VM without GNS3
• Step 2 build the VM by adding GNS3

This will speedup the release process because we just need to add GNS3 and will avoid the need to debug the VM build for a minor release (I just lost a lot time due to an ubuntu mirror close from home out of sync...).