Add run_single_ostf_test method in nailgun client

To get possibility invoke separate ostf test
in system_test method run_single_ostf_test
was added

Change-Id: I2118aa9b17c47fc55e6e3f54f3b4c2307d2ce1b3
Closes-Bug: #1258499
Set KVM usage false by defualt

As temporary fix to get instances Active,
we need use qemu in sys tests
Change KVM_USE value in setting.py by default
false value, if os environ is not defined
Add else part in fuel_web client to
verify if KVMS_USE false, update OS setting to qemu

Change-Id: Ifc817104d6b8a6d3a5de27c10c07afcdc92f283f