Trainer add args doesn't add default root dir
🐛 Bug

When using parser = Trainer.add_argparse_args(parser), it's supposed to put all Trainer's arguments in the argparse with default values. Though currently it doesn't add default_root_dir and you get the error:

'Namespace' object has no attribute 'default_root_dir'

It does add default_save_path which is deprecated.
To Reproduce
Code Sample
import argparse
from pytorch_lightning import Trainer

parser = argparse.ArgumentParser(description='demo')
parser = Trainer.add_argparse_args(parser)
args = parser.parse_args()

print(args.default_root_dir)
A similar unit test could also be made, if not there already.
Environment
* CUDA:
        - GPU:
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
                - GeForce RTX 2080 Ti
        - available:         True
        - version:           10.1
* Packages:
        - numpy:             1.18.1
        - pyTorch_debug:     False
        - pyTorch_version:   1.4.0
        - pytorch-lightning: 0.7.3
        - tensorboard:       2.2.0
        - tqdm:              4.45.0
* System:
        - OS:                Linux
        - architecture:
                - 64bit
                -
        - processor:         x86_64
        - python:            3.6.7
        - version:           #75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019