fix: maas and postgres to use `_helpers.tpl` rather than `helm-toolkit`

[v1k0d3n]

as part of the migration of maas and postgresql (PR #4), and to allow these charts to function outside of openstack-helm (remove the built in functions of helm-toolkit), _helpers.tpl should be leveraged. on their own, these charts will not work. this will increase the usability of these charts beyond openstack-helm (our needs).

REQUIREMENT: This issue requires #4 be merged in first (before the work on this issue can be started).

[intlabs]

@v1k0d3n I could fix this this morning if you like? Should only take 10 mins or so.