scylla_node: preserve node SMP across restarts

ScyllaNode.start() was resetting _smp to the default value (2) unless
the test explicitly called ScyllaNode.set_smp(). This caused nodes
started with a different "--smp" value from command lines and then
restarted to lose their previous CPU configuration:

```
cluster.start(jvm_args=["--smp", "1"], wait_for_binary_proto=True)
node1.stop(gently=False)
node1.start(wait_for_binary_proto=True)
```
The last command started a node with --smp=2.

Keep the existing _smp value on restart, and only update it when
explicitly requested (via set_smp() or an explicit --smp override
in start options).

Fixes: https://scylladb.atlassian.net/browse/SCYLLADB-521

Author: paszkow

ccmlib/scylla_node.py

@@ -701,19 +701,21 @@ def start(self, join_ring=True, no_wait=False, verbose=False,
         if not self._mem_mb_set_during_test and '--memory' in cmd_args:
             self._memory = self.parse_size(cmd_args['--memory'][0])
 
+        # use '--smp' in jvm_args if was not set by the test
+        if not self._smp_set_during_test and '--smp' in cmd_args:
+            self._smp = int(cmd_args['--smp'][0])
+
         ext_args = env_args
         ext_args.update(cmd_args)
         for k, v in ext_args.items():
-            if k == '--smp':
-                # get smp from args if not set by the test
-                if not self._smp_set_during_test:
-                    self._smp = int(v[0])
-            elif k != '--memory':
-                if v and all(v):
-                    for val in v:
-                        args += [k, val]
-                else:
-                    args.append(k)
+            if k in ('--memory', '--smp'):
+                continue
+
+            if v and all(v):
+                for val in v:
+                    args += [k, val]
+            else:
+                args.append(k)
 
         args.extend(translated_args)