package-installs: fix error case for Python 2.6
package-installs: fix error case for Python 2.6

subprocess.CalledProcessError in Python 2.6 does not have the 'out'
parameter for __init__, so pass only two of them and manually set
'output' in that case.

Fixes/improves commit 7f410aaff211a1d84f63e7ddf63b0a8badebea0a.

Change-Id: I279bdf433b1272a9c3af4d66a2a52c78a7ac5de2

takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,zioproto/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,rdo-management/diskimage-builder,rdo-management/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder