use std importer configuration keys; pass max bandwidth & concurrency for downloads.
remove unused import.
Add model test.
updated node unit tests.
Merge branch 'master' into nodes-bandwidth-concurrency
add bandwidth and concurrency options support to nodes extensions.
remove unused import
add --max-downloads, max-speed to nodes command user-guide.
better user-guide formatting.
Merge pull request #548 from pulp/nodes-bandwidth-concurrency

Support passing max-bandwidth and max-concurrency options for node synchronization.