Provide completion candidates for remote containers over a multi-hop

Provide completion candidates for remote containers over a multi-hop

* lisp/net/tramp-container.el (tramp-container--completion-function):
Set default directory to last hop.
(tramp-set-completion-function): Don't use executable-find for
container program since it might not be running locally.

* lisp/net/tramp.el (tramp--last-hop-directory): Add new variable.
(tramp-completion-handle-file-name-all-completions): Use container
host directory to execute container program on remote host.
(tramp-set-completion-function): FIXME: for now, don't constrain
allowable completion methods to only those present on the local system.
(tramp-completion-remote-containers): Add customize option to
provide completion candidates for containers running on remote hosts.