openslide reader not tested/integrated into docker image

[wyli]

Describe the bug
the openslide installation steps are currently not implemented in the testing and docker building pipelines
https://github.com/Project-MONAI/MONAI/runs/2058917545?check_suite_focus=true#step:10:5509

need to follow https://github.com/openslide/openslide-python#installation

[wyli]

need at least apt-get install python3-openslide before running any python code. the command might be too heavy to run for each PR commit, but probably need for the nightly test and the dockerfile cc @behxyz @Nic-Ma what do you think?

[bhashemian]

@wyli, I agree! It's too heavy for each PR and also it requires to configure tzdata if it is not already configured on the docker container. Nightly tests only seems to be a fair compromise. Also apt-get install libopenslide0 has less dependencies than pytho3-openslide and should suffice our purposes.

[bhashemian]

OR can't we create another docker image on top of nvcr.io/nvidia/pytorch:20.03-py3 (or any later version that we are going to use) with all these system dependencies installed, and just install python libraries each time?