dpstack: adding suffix for multiple usecase runs

Adding suffixes in all usecase. These suffixes will help in
simultaneous running of different usecases in parallel.
Sockfile and pidfile creation is done based on suffix given from
arguments. Default suffix is username

Signed-off-by: Sanjyot Vaidya <Sanjyot.Vaidya@arm.com>
Change-Id: Idfb8e9917e703c9e7a107ec23adf84af64c80738