Allow overriding default QContactManagerEngine via env var

Add support for overriding the default QContactManagerEngine via the
QTCONTACTS_MANAGER_OVERRIDE environment variable.

Port forward from QtMobility Contacts, commit 47079b33a0c1b5fe2301bb527cca12df13f3b1d6

Change-Id: I073904e0d61065719a7eacc8437943e2e157f6dc
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>