Remove vttop / simplify build structure

[morgo]

Feature Description

As discussed on the monthly Vitess call, we should simplify the build structure of Vitess. So currently there is VTROOT, which requires a subdirectory of structure src/vitess.io/vitess inside it.

VTROOT + this src/vitess.io/vitess = VTTOP.

The proposal is to set VTROOT to what is currently VTTOP, and then eliminate VTTOP. This will allow someone to check out and build Vitess as follows:

git clone git@github.com:vitessio/vitess.git
cd vitess
make build

cd examples/local
./101_initial_cluster.sh