Check if command is implemented in a plugin before executing plugin code

Because of how plugin commands are implemented, their output can be incredibly verbose. Rather than executing even the `set -eo pipefail` parts of a plugin, we immediately check if the command is implemented by a plugin. If it is not, then we continue on as normal.

One side-effect of this change is that plugin commands need to be duplicated again:

- once in the command array
- once for the actual body of the command
- once in the help output

This is also quite hackish, and probably not the best way to decrease trace output. Note that we drop approximately 2k lines worth of logs with this change.

plugins/shell/commands

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+[[ " shell help shell:help " == *" $1 "* ]] || exit $DOKKU_NOT_IMPLEMENTED_EXIT
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 
 case "$1" in