[SPARK-7771] [SPARK-7779] Dynamic allocation: lower default timeouts further

The default add time of 5s is still too slow for small jobs. Also, the current default remove time of 10 minutes seem rather high. This patch lowers both and rephrases a few log messages.

Author: Andrew Or <andrew@databricks.com>

Closes #6301 from andrewor14/da-minor and squashes the following commits:

6d614a6 [Andrew Or] Lower log level
2811492 [Andrew Or] Log information when requests are canceled
5fcd3eb [Andrew Or] Fix tests
3320710 [Andrew Or] Lower timeouts + rephrase a few log messages