Isolate tests from HTTP_PROXY, OS_REGION_NAME env vars

CLI and shell tests call into a mock httpretty REST service.  If
HTTP_PROXY is set, urllib2/requests follows it and fails to hit the
mocked httpretty service as expected.

Also: The shell tests assumed OS_REGION_NAME was unset, so also clear
this.

Change-Id: I9a3f4981ee5c32203f9127e9eb8eefe5efd1b6dc
Closes-Bug: #1367102
Closes-Bug: #1367105
Narrow down except clause

To avoid masking brainfarts like the ones that led to
bug report below.

Related-bug: #1368835

Change-Id: I028b779e580da7719925a8e127f48bbe1aec211e
Merge "Narrow down except clause"
Merge "Isolate tests from HTTP_PROXY, OS_REGION_NAME env vars"