Fix error on 'Devices Health History' page in django 1.10
Fix error on 'Devices Health History' page in django 1.10

In django 1.10 select_related() prohibits non-relational fields for nested
relations.

See https://docs.djangoproject.com/en/1.10/releases/1.10/#select-related-prohibits-non-relational-fields-for-nested-relations

Change-Id: If0e4ffc567b2f7bf3a57e315ce06917fccfd3ef2

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server