TBG: Add Defaults via Env for -s|-t

This feature shortens the `tbg` call dramatically without harming its generality.

The `-s | --submit` option is the batch system and unique for each machine. That is a perfect candidate for a `picongpu.profile` or a general `.profile` setting (as a default).

The `-t | --tpl` file is useful if you have various queues or debug templates. 99% of the users always submit to the same queue and would love a default, too. Developers or power users with varying queues can still use `-t` to overwrite the default.

If `TBG_SUBMIT` or `TBG_TPLFILE` are not set, the default values are still `""` which will result in a standard error from `tbg`.
Should we add a separate error message for the case that there is no environment variable `TBG_TPLFILE` and the option `-t` without parameter is used, else the error message looked like this.
`The given tpl file  does not exist.`