Change libcontainer to drop all capabilities by default. Only keeps
those that were specified in the config. This commit also explicitly
adds a set of capabilities that we were silently not dropping and were
assumed by the tests.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #5810 from vmarmol/drop-caps

Change libcontainer to drop all capabilities by default.