[tests] Move stop_node and start_node methods to BitcoinTestFramework

This commit moves functions start_node, start_nodes, stop_node and
stop_nodes functions into the BitcoinTestFramework class. It also moves
the bitcoind_processes dict and coverage variables into BitcoinTestFramework.

test/functional/keypool.py

@@ -18,7 +18,7 @@ class KeyPoolTest(BitcoinTestFramework):
         
         # Encrypt wallet and wait to terminate
         nodes[0].encryptwallet('test')
-        bitcoind_processes[0].wait()
+        self.bitcoind_processes[0].wait()
         # Restart node 0
         nodes[0] = self.start_node(0, self.options.tmpdir)
         # Keep creating keys