[env] Set default QtMobility Contacts engine

If the qtcontacts-sqlite (org.nemomobile.contacts.sqlite) plugin for
QtMobility Contacts API exists, it should be used by default.
A recent patch to QtMobility adds support for enabling overriding the
platform default manager engine via the QTCONTACTS_MANAGER_OVERRIDE
environment variable.
Merge pull request #1 from chriadam/master

Use qtcontacts-sqlite engine by default