Fixed bug with listing installed subcommands.
Fixed bug with listing installed subcommands.

The command names of installed subcommands are now embedded under a
'command' key instead of living at the top level. This commit fixes the
'merge_installed()' function to index into the dictionary of subcommands
properly to extract the names now that this change has been introduced.

mesosphere/dcos-cli,dcos/dcos-cli,mesosphere/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,mesosphere/dcos-cli