Respects TOCI_REPOREF_* variables for tripleo subprojects too

If TOCI_REPOREF_* variables are used to set specific commit hash
for basic tripleo repos (diskimage-builder, tripleo-image-elements...),
then respect this variable when fetching repos in toci_git.sh.

Change-Id: I2c32ce91bc30992a072ab9ccff6ec0e8aa2c3242