Add description to the help of enable_instance_password
Add description to the help of enable_instance_password

enable_instance_password in tempest.conf is a little
confusing to users who are not so familier with nova.
So this is to add a simple guide for setting its value
(just the same as the value in nova.conf)

Change-Id: I1ee732c3673cac1b64dcadeb1f6f65e3691fd9c6

Tesora/tesora-tempest,masayukig/tempest,cisco-openstack/tempest,Tesora/tesora-tempest,masayukig/tempest,Juniper/tempest,sebrandon1/tempest,cisco-openstack/tempest,openstack/tempest,vedujoshi/tempest,sebrandon1/tempest,Juniper/tempest,openstack/tempest,vedujoshi/tempest