Optimize some of compute/manager's periodic tasks' DB queries

Most of these tasks do not need to fetch instances with a full
complement of metadata items. Make them say "no thanks" to the DB API
to reduce the amount of work that needs to be done. Note some
refactoring was done in the _get_instance_nw_info() method to pull
this information if it ended up being necessary. The cases where this
would happen are error-cleanup scenarios, and thus taking the extra
hit in that case is an overall win for not having to do it all the
time.

Related to bug 1164737

Change-Id: Ic7d6290c0b7edf84cbf8bdc64e29f206619f97e5
Merge "Optimize some of compute/manager's periodic tasks' DB queries"