cmd/tsuru-base: add a new flag to the command

The flag -t will be used to provide the team that owns the service
instance being created. It's optional.

Closes #698.
cmd/tsuru-base: use a pointer receiver in ServiceRemove
cmd/tsuru-base: service-remove is not a guessing command

Guessing commands are commands that have the -a/--app parameter and can
try to guess it from the git remote named "tsuru".