Modified dismiss_keyborad() method.

- for gaia keyboard change. (Bug 911737)

```
======================================================================
ERROR: test_clock_set_alarm_label (test_clock_create_new_alarm.TestClockCreateNewAlarm)
Set label of the new alarm
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/lib/jenkins/jobs/B2G.master.unagi.gaiatest/workspace/.env/local/lib/python2.7/site-packages/marionette_client-0.5.36-py2.7.egg/marionette/marionette_test.py", line 132, in run
    testMethod()
  File "/var/lib/jenkins/jobs/B2G.master.unagi.gaiatest/workspace/gaiatest/tests/clock/test_clock_create_new_alarm.py", line 51, in test_clock_set_alarm_label
    new_alarm.type_alarm_label(alarm_label_text)
  File "/var/lib/jenkins/jobs/B2G.master.unagi.gaiatest/workspace/gaiatest/apps/clock/regions/alarm.py", line 32, in type_alarm_label
    self.dismiss_keyboard()
  File "/var/lib/jenkins/jobs/B2G.master.unagi.gaiatest/workspace/gaiatest/apps/base.py", line 147, in dismiss_keyboard
    self.wait_for_condition(lambda m: m.find_element(By.CSS_SELECTOR, '#keyboards iframe').location['y'] == 480)
  File "/var/lib/jenkins/jobs/B2G.master.unagi.gaiatest/workspace/gaiatest/apps/base.py", line 98, in wait_for_condition
    raise TimeoutException(message)
TEST-UNEXPECTED-FAIL | test_clock_create_new_alarm.py TestClockCreateNewAlarm.test_clock_set_alarm_label | TimeoutException: Condition timed out
----------------------------------------------------------------------
Ran 2 tests in 139.939s
```