Handle S3 store optional parameters
Handle S3 store optional parameters

S3 store has two optional parameters that may be omited.

Use `CONFIG.get(…)` to get their value or None rather than `CONFIG[…]`
which fails.

Change-Id: I5c084845eba40045cd947071963819fb0fefd6b9

redhat-cip/dci-control-server,redhat-cip/dci-control-server