Suppress the error message in cleanup-env

"default" pool may not exist before cleaning. Suppress the possible
error message:
error: failed to get pool 'default'
error: Storage pool not found: no storage pool with matching name 'default'

Change-Id: I80b4e3732888c7ebadd03b7792a83dd2a6c411d5
Close-Bug: #1261257
Merge "Suppress the error message in cleanup-env"