Add cmake flag to determine testnet vs mainnet build

If you want to build Mainnet node

cmake -DBOOST_ROOT="$BOOST_ROOT" -DCMAKE_BUILD_TYPE=Release

If you want to build Testnet node

cmake -DBOOST_ROOT="$BOOST_ROOT" -DCMAKE_BUILD_TYPE=Release -DBUILD_PEERPLAYS_TESTNET=1