Use constants for these strings instead of freezing them.
Use constants for these strings instead of freezing them.

Reworked #4371 based on: https://github.com/tenderlove/psych/pull/249

rsevilla87/manageiq-gems-pending,aljesusg/manageiq-gems-pending,rsevilla87/manageiq-gems-pending,aljesusg/manageiq-gems-pending