Improve --debug logging output

This change makes the following improvements to the output of a heat
command with the --debug option set:
* Silences spurious logging from iso8601 and urllib3.connectionpool
* Removes the line number from the logging format, since the intent
  of --debug is to show the user what network requests are occuring.

Change-Id: Iccbae01a1a5b85db0770a4f56c3b48788dc3008a
Closes-bug: #1324470