Use gevent sleep for switching context

`time.sleep` was changed to `gevent.sleep` to allow current greenlet to
sleep and others to run. When `time.sleep` is used, greenlet's context
is not changed from the main process to the test greenlet.

As a result of this, there is no data received by the main process while
hanging the tarantool server process and the test is fallen down by the
common timeout (NO_OUTPUT_TIMEOUT). Moreover, the process is not killed
by test-run.

Using `gevent.sleep` makes the test fall down by the test timeout and
kill the farantool server process.

Part of #276

Author: VitaliyaIoffe

lib/tarantool_server.py

@@ -448,7 +448,7 @@ def seek_wait(self, msg, proc=None, name=None):
         while True:
             if os.path.exists(self.path):
                 break
-            time.sleep(0.001)
+            gevent.sleep(0.001)
 
         with open(self.path, 'r') as f:
             f.seek(self.log_begin, os.SEEK_SET)
@@ -459,7 +459,7 @@ def seek_wait(self, msg, proc=None, name=None):
                         raise TarantoolStartError(name)
                 log_str = f.readline()
                 if not log_str:
-                    time.sleep(0.001)
+                    gevent.sleep(0.001)
                     f.seek(cur_pos, os.SEEK_SET)
                     continue
                 if re.findall(msg, log_str):
@@ -1123,7 +1123,7 @@ def wait_until_started(self, wait_load=True):
                 if e.errno == errno.ECONNREFUSED:
                     color_log(' | Connection refused; will retry every 0.1 '
                               'seconds...')
-                    time.sleep(0.1)
+                    gevent.sleep(0.1)
                     continue
                 raise