[SPARK-10825] [CORE] [TESTS] Fix race conditions in StandaloneDynamicAllocationSuite

Fix the following issues in StandaloneDynamicAllocationSuite:

1. It should not assume master and workers start in order
2. It should not assume master and workers get ready at once
3. It should not assume the application is already registered with master after creating SparkContext
4. It should not access Master.app and idToApp which are not thread safe

The changes includes:
* Use `eventually` to wait until master and workers are ready to fix 1 and 2
* Use `eventually`  to wait until the application is registered with master to fix 3
* Use `askWithRetry[MasterStateResponse](RequestMasterState)` to get the application info to fix 4

Author: zsxwing <zsxwing@gmail.com>

Closes #8914 from zsxwing/fix-StandaloneDynamicAllocationSuite.