Update CONTRIBUTING.rst to cover conditionals
Update CONTRIBUTING.rst to cover conditionals

Performing requested update of CONTRIBUTING.rst to convey
boolean casting to contributers.

Change-Id: I1e8dc67ce765c4996ff671a7299aa16379c00432

EntropyWorks/bifrost,openstack/bifrost,bcornec/bifrost,openstack/bifrost,bcornec/bifrost,EntropyWorks/bifrost