#74 Add env var to helm chart to fix 500 internal errors

As documented in the values.yaml file:

SQLALCHEMY_ENABLE_POOL_PRE_PING value, controls whether the database connection that's in the pool will be checked by pinging before being used or not.
Turning this on prevents 500 Internal Server Errors that some people are facing.
This env var is related to https://github.com/getredash/redash/issues/4675 and the fix is already merged @ https://github.com/getredash/redash/pull/4741
look here for more information: https://docs.sqlalchemy.org/en/13/core/pooling.html#sqlalchemy.pool.Pool.params.pre_ping

Default should be set to true.