add support for source=swift

While we are running both filesystem and swift in parallel, it
would be good to be able to bypass the filesystem to test the
difference between swift and volume performance.

Do this with the addition of a source= cgi parameter. Currently
source=swift is the only thing that will change behavior. Internally
we default it to all.

Refactoring of the cgi param parsing is done to make it cleaner
code and not copy/paste the cgi param parsing a 3rd time.

Change-Id: Ic653bad9a0f74fb5714ff7ccfc7da8d790dcff8b
Merge "add support for source=swift"