openib btl: add Soft iWARP device to the ini file

This enables IBM's software iWARP provider.  With this driver you can
run iWARP/RDMA over any ethernet NIC.  Useful for testing OMPI RDMA
logic without requiring an expensive RDMA adapter/infrastructure.

The Soft iWARP code is at: https://www.gitorious.org/softiwarp

Cherry-picked from open-mpi/mpi@7316a887546acad2010c27087a403bf7dfe04dc6
Merge pull request #75 from larrystevenwise/v1.8

openib btl: add Soft iWARP device to the ini file