Poll for replica read_only status in test

There is a time lag for the read_only system variable in the
detached replica to change from true to false, causing the test
to fail many times.Make the test_slave_is_not_read_only
poll unitl the value changes.

Change-Id: I6191cb7f85e18b30a16383675490a6186185ba5b
Closes-Bug: #1393914
Merge "Poll for replica read_only status in test"