Add --exp to dev command

[NicolasMahe]

It's REALLY annoying to deactivate the network by removing the flag in the dev script.
So now we can pass the flag --experimental-network to enable the network feature.
This is only a temp solution until the network is the default solution and fully implemented!

This PR is a simplified version of #1262.

./dev # no network
./dev --exp # with network
./dev --exp --validator # with network and node is validator

[krhubert]

I like it because it was annoying to me as well.

If it's temporary, can we make a short flag for this? It will be really painful to write --experimental-network if one start working on the network. -n for network? what do you think?

[antho1404]

I'm fine to make it shorter and fine to keep it like it anyway I use the reverse-i-search all the time so it's will not change anything