Fix NailgunExecutor to support more than one connect attempt

In 73e25e7, the `WAIT_INTERVAL` field of `ProcessManager` was renamed to
`WAIT_INTERVAL_SEC`, except for one location in `NailgunExecutor` which broke as
a result.  This causes the `try_connect` method to fail if one connection
attempt fails, regardless of what the user supplies as the
`--nailgun-connect-attempts` flag on the command line.

Testing Done:
I discovered this issue by using `pylint` and then manually verifying that the warning was correct:

```
$ pylint -E -d no-self-argument src/python/pants/java/nailgun_executor.py
No config file found, using default configuration
************* Module pants.java.nailgun_executor
E:229,17: Instance of 'NailgunExecutor' has no 'WAIT_INTERVAL' member
(no-member)
```

After fixing the issue the `pylint` warning disappears:

```
$ pylint -E -d no-self-argument src/python/pants/java/nailgun_executor.py
No config file found, using default configuration
```

Bugs closed: 2196

Reviewed at https://rbcommons.com/s/twitter/r/2822/

src/python/pants/java/nailgun_executor.py

@@ -226,7 +226,7 @@ def ensure_connectable(self, nailgun):
         sock.close()
 
       attempt_count += 1
-      time.sleep(self.WAIT_INTERVAL)
+      time.sleep(self.WAIT_INTERVAL_SEC)
 
   def _spawn_nailgun_server(self, fingerprint, jvm_options, classpath, stdout, stderr):
     """Synchronously spawn a new nailgun server."""