This updates the prometheus chart to include the pod
security context on the pod template. This changes the pod's
user from root to the nobody user instead
This also adds the container security context to explicitly set
allowPrivilegeEscalation to false and readOnlyRootFilesystem to true
Change-Id: I2a3a4b77d9b25c086dc23b4fd66dca92872c422d
f25e458515