tests: Adding parallelism helpers in ceph-helpers.sh

tests: Adding parallelism helpers in ceph-helpers.sh

This commit introduce two new functions in ceph-helpers.sh to ease
parallelism in tests.

It's based on two functions : run_in_background() & wait_background()

The first one allow you to spawn processes or functions in background and saves
the associated pid in a variable passed as first argument.

The second one waits for thoses pids to complete and report their exit status.
If one or more failed then wait_background() reports a failure.

A typical usage looks like :

 pids1=""
 run_in_background pids1 bash -c 'sleep 5; exit 0'
 run_in_background pids1 bash -c 'sleep 1; exit 1'
 run_in_background pids1 my_bash_function
 wait_background pids1

The variable that contains pids is local making possible to do nested calls of
thoses two new functions.

Signed-off-by: Erwan Velu <erwan@redhat.com>