Bug 1571910: Set placeholder cacheversion before upgrade

Allow get_config to supply a default so that system can handle misisng
config values during upgrade from older sites that are meant to be set
duing upgrade, eg 'cacheversion'

behatnotneeded - upgrade issue

Change-Id: Ieaea390d57c18f3bc3e0808026ec1e76636bcc15
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit d7c297f1)