Comet logger overrides COMET_EXPERIMENT_KEY env variable

[Vozf]

After #2553 there is a changed logger behavior. It starts using COMET_EXPERIMENT_KEY. But it doesn't respect it if it is set already.
So the bug is in the following.
I already set this variable
Then logger overwrites my value here https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/loggers/comet.py#L189
Then it deletes this variable at all here https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/loggers/comet.py#L215
This way it ignores my variable and deletes it at all later
Moreover in version function it also ignores my set variable
I will create a pull request to fix it