Change the nginx app version to a set tag
Change the nginx app version to a set tag

Want to avoid moving tags like using 'stable'. Therefore,
specify the specifc nginx version/tag.

Update from comment review:
- https://github.com/helm/helm/pull/5662#discussion_r280122531

Signed-off-by: Martin Hickey <5a111d7c144d6e07073f7da1705506e1d8bdbf43@ie.ibm.com>

adamreese/deployment-manager,adamreese/deployment-manager,adamreese/deployment-manager,michelleN/deployment-manager,adamreese/deployment-manager,helm/helm,michelleN/deployment-manager,michelleN/deployment-manager,adamreese/deployment-manager,bacongobbler/helm,michelleN/deployment-manager,adamreese/helm,helm/helm,adamreese/helm,bacongobbler/helm