Limit InstanceList join to system_metadata in os-simple-tenant-usage

The simple-tenant-usage API is calling get_active_by_window_joined with
all of the default instance attribute fields when really only
system_metadata is needed (to get the related flavor). That adds
unnecessary load times on the DB query and data processing when
constructing the instance list, especially given this is a query over a
time range of instances, so as you add more instances to scale it's
going to slow down.

This simply limits the expected_attrs in the get_active_by_window_joined
call to system_metadata so we don't load the other unnecessary
attributes.

Closes-Bug: #1383469

Change-Id: Ic77d51cd45730ca664e68cfe0cb1671b5442d4a9
Enable pep8 on ./tools directory

This patch fixes the pep8 issues of the tool scripts and
configures tox to run pep8 on the tools directory.

Two hacking rules have been modified to ignore the tool directory as both
rules are checking oslo usage which is not critical in case of tools
- N310 use timeutils instead of datetime module
- N324 use jsonutils instead of json module

The tools/xenserver directory is still excluded from pep8 as it contains
scripts that are expected to run on dom0 with python2.4 therefore the
following rules cannot be meaningfully followed
- H231  Python 3.x incompatible 'except x,y:' construct
- H233  Python 3.x incompatible use of print operator

Change-Id: Icfd9e708a2ca1e421676b410e41807b2e630718f
Merge "Limit InstanceList join to system_metadata in os-simple-tenant-usage"
Merge "Enable pep8 on ./tools directory"