Bootstrap Chain

[nunojusto]

Problem

Solving issue https://github.com/CityOfZion/neo-local/issues/87

Solution

A switch in the .env file to choose what chain to bootstrap/use. Those images must be already built.

Checklist

• [x] I have ran the tests locally.
• [x] I branched off of the develop branch and not master.
• [x] I did not change the VERSION file.

[nunojusto]

Close PR https://github.com/CityOfZion/neo-local/pull/106 and opened this for the same stuff. But a lot cleaner...