Revert "Disable eventlet monkey-patching of DNS"

This reverts commit 6219f94b3a1103a16bd5f16ed8ffdd41a85280e1.

This commit seems to cause keystone auth to hang when using
Keystone w/ Swift.

Fixes LP Bug #1178646.

Change-Id: Ieb94a29b3129847c84a02c8a526611f125f21ea0