Fix postinstall for npm job
Fix postinstall for npm job

npm jobs uses python3.5, but pip uses python2.7 to install horizon.
This fixes this issue.

Change-Id: Ib82a7461949e42eef5f155b13e7efbd7f65139f5

openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard