costa: use conda instead of venv to support some packages for ppc64le

[yorkie]

As #603 described:

The same problem is found here: https://stackoverflow.com/questions/60825109/tensorflow-installation-fails-on-powerpc-ppc64le, which is caused by no suitable tensorflow package for ppc64le, we might need to change to use conda instead venv to support Pipcook on this type of machine.

Currently, we are using the venv as the Python packages which are not suitable for some platforms like ppc64le, and the conda is able to address this issue.

[yorkie]

The steps for init and install packages:

1. .miniconda/bin/conda create -p {fullpath}
2. .miniconda/bin/conda install -p {fullpath} {package@ver}

And some unresolved problems:

• How to set up the conda-env directory?
  • Resolved via -p.
• How to set up the mirror for conda?
  • Resolved via the custom channel, see https://mirrors.tuna.tsinghua.edu.cn/help/anaconda/.
• How to use the conda-env without conda activate?

[yorkie]

We can use conda API to avoid forking an unneeded process and friendly to the scheduler.