Allow user deployment startup probes to be disabled in helm charts

Summary:
Additionally, set the startup probe's default exec command based on the
existence of the startup probe's exec and not the existence of the
liveness probe's exec.

Test Plan:
unit
integration

Reviewers: rexledesma, johann, dgibson

Reviewed By: rexledesma

Differential Revision: https://dagster.phacility.com/D7568