Make testr preserve existing OS_* env vars values

Values for OS_TEST_TIMEOUT, OS_STDOUT_CAPTURE and OS_STDERR_CAPTURE
environment variables are fixed in .testr.conf now. It should be possible
to override them for each tox/runtests.sh call or for the current shell
session without making changes to .testr.conf.

Fixes bug 1182416.

Change-Id: I6c7839fa78b53ac8ea358ce0143a00931da96a9e
Clean up failed image transfers in instance spawn

Boot one instance with powervm driver, will transfer the image
file to host. Need clean up failed image transfers that may occur
during instance spawn.

Fixes bug #1173017

Change-Id: Iff5ecd32e201333567a6a4de1d3796e78ea5dc0c
Merge "Clean up failed image transfers in instance spawn"
Merge "Make testr preserve existing OS_* env vars values"