Adds information on parallel test execution
Adds information on parallel test execution

Since tests can be run with py.test in a parallel manner,
let's reflect the instructions in the Developer Guide.

Change-Id: I03a2cbfe5e9e1a8e57afcd219cd0df0bd9c9fae3
Closes-Bug: 1391873
Closes-Bug: 1396623

prmtl/fuel-web,stackforge/fuel-web,SmartInfrastructures/fuel-web-dev,nebril/fuel-web,nebril/fuel-web,SmartInfrastructures/fuel-web-dev,huntxu/fuel-web,stackforge/fuel-web,SmartInfrastructures/fuel-web-dev,eayunstack/fuel-web,zhaochao/fuel-web,huntxu/fuel-web,huntxu/fuel-web,nebril/fuel-web,eayunstack/fuel-web,zhaochao/fuel-web,zhaochao/fuel-web,zhaochao/fuel-web,eayunstack/fuel-web,prmtl/fuel-web,stackforge/fuel-web,zhaochao/fuel-web,SmartInfrastructures/fuel-web-dev,nebril/fuel-web,huntxu/fuel-web,SmartInfrastructures/fuel-web-dev,eayunstack/fuel-web,prmtl/fuel-web,nebril/fuel-web,eayunstack/fuel-web,prmtl/fuel-web,prmtl/fuel-web,huntxu/fuel-web