Keep environment for sudo

To be able to set some other environment variables even if dokku gets
called from a different user we have to keep the environment with sudo
intact, so i added the -E option.
Replaced hard coded plugins path

To be able to switch the plugin path i have removed the hard coded
plugin path from the catch all section of the switch within the main
dokku executable.
Merge pull request #777 from feature/improve-core