Replace overcommit configuration file name with constant

Make referencing this name consistent everywhere in code.

We don't use the constant in specs because we want to be made aware of
what changes if we were to ever change this constant (unlikely to
happen).

Change-Id: Ieeda0d8d59cd9719523bf72759510d48b3cf57aa
Reviewed-on: http://gerrit.causes.com/40981
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>