Fixes in test harness to make all them work at jenkins

Basically:

* Move mongo command direct invokation to a shared function in harnessFunction.sh named mongoCmd(). I don't know why `mongo --quiet` fails when executed inside a jenkins jobs, but my workaround seems to work without readability impact in the code in my opinion.
* Split contextElements in updateContext in some subcribeContext ONCHANGE test (append and update), to avoid race conditions in notifyContextRequest ordering.

I have tested that this works using a debug jenkins job (see http://ci-fiware-01/jenkins/view/Orion/job/orion-debug-harness/11/console).

We could have some conflict when merging cm_forwards_psb_ok.test and cm_forwards_psb_using_convops_ok.test, that are also touched in other parallel PR at this point in time (PR #160) but I hope the be easily solvable.

I think this PR doesn't implies any change in LAST_CHANGES file.