cfg: set default values for swift3 config parameters

This makes it clear that what kinds of values are allowed for Swift3 config.
The sample config file is also added to document those parameters.

The default value of 'location' is set to 'US' like AWS S3.

Change-Id: Ifa18346208863cffb05862e062043ecdb35341b7
Merge "cfg: set default values for swift3 config parameters"