As a relief valve for times of storage pressure on the receiving end,
add a flag that can be set to skip log uploads for sucessful jobs.
It uses the same logic as emit-ara-html.
zuul_site_upload_logs == true (the default) will always upload logs
zuul_site_upload_logs == false will never upload logs
zuul_site_upload_logs == 'failure' will only upload failure logs
Setting this variable should be done in site variables.
Change-Id: I2a55730976520b75dcc98f86f34b7929f9243e62
3d02630d1b