Add --rebuild option for start command

The --rebuild option can be used to rebuild an existing container. Before this
change, we would always remove the container after it finished (via the --rm
option to the 'docker run' command). That option is no longer used and the
user must explicitly request a rebuild of a service container. The names of
the services can be listed after the --rebuild option, or, if no names are
listed, all containers will be rebuilt.

Examples:

   # rebuild all containers
   dockstack start --rebuild

   # rebuild only nova and glance containers
   dockstack start --rebuild nova glance