clients/besu: fix clique mining (ethereum#730)

Reversed the logic to deploy clique key to bootnode only.
Original change was to avoid deploying the same clique key to 2 different
nodes. Ended up stopping the deployment of the clique key to the first node,
causing test that rely on a single node to mine to fail due to timeout.

clients/besu/besu.sh

@@ -99,7 +99,7 @@ if [ -d /blocks ]; then
     IMPORTFLAGS="$IMPORTFLAGS $blocks"
 fi
 
-if [ "$HIVE_BOOTNODE" != "" ]; then
+if [ "$HIVE_BOOTNODE" == "" ]; then
     # Configure mining.
     if [ "$HIVE_MINER" != "" ]; then
         FLAGS="$FLAGS --miner-enabled --miner-coinbase=$HIVE_MINER"