[SPARK-10955] [STREAMING] Add a warning if dynamic allocation for Streaming applications

Dynamic allocation can be painful for streaming apps and can lose data. Log a warning for streaming applications if dynamic allocation is enabled.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #8998 from harishreedharan/ss-log-error and squashes the following commits:

462b264 [Hari Shreedharan] Improve log message.
2733d94 [Hari Shreedharan] Minor change to warning message.
eaa48cc [Hari Shreedharan] Log a warning instead of failing the application if dynamic allocation is enabled.
725f090 [Hari Shreedharan] Add config parameter to allow dynamic allocation if the user explicitly sets it.
b3f9a95 [Hari Shreedharan] Disable dynamic allocation and kill app if it is enabled.
a4a5212 [Hari Shreedharan] [streaming] SPARK-10955. Disable dynamic allocation for Streaming applications.