Fixed compiler warning in values tests.

Fixed compiler warning in values tests.

Added initial draft of networking user-doc.

Review: https://reviews.apache.org/r/38963

Added `image_providers` flags to configuration.md.

Review: https://reviews.apache.org/r/38749

Updated configure.ac to 0.21.2

Updated configure.ac to 0.21.2

Updated configure.ac to 0.22.2

Updated configure.ac to 0.22.2

Updated configure.ac to 0.23.1

Updated configure.ac to 0.23.1