[BACKPORT]Update Ubuntu images used on CI to 20.04 (#19588)

[access2rohit]

• Update Ubuntu images used on CI to 20.04

This helps ensure MXNet to work well on recent Linux distributions (while ensuring it continues to work well on ancient distributions based on the CentOS7 CI pipeline)

• Preserve Ubuntu 18.04 images for TensorRT pipeline as NVidia failed to make TensorRT available for Ubuntu 20.04

• Temporarily disable NVML on CI

[2020-12-03T18:33:10.380Z] OSError: /work/mxnet/python/mxnet/../../build/libmxnet.so: undefined symbol: nvmlDeviceGetComputeRunningProcesses_v2

backport #19588 as a part of effort #19911

[mxnet-bot]

Hey @access2rohit , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

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CI supported jobs: [website, miscellaneous, unix-gpu, centos-cpu, windows-gpu, unix-cpu, sanity, windows-cpu, centos-gpu, edge, clang]

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Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[access2rohit]

@leezu @josephevans please review

[access2rohit]

Not required..... Closing

[leezu]

You may still want to update some of the containers that are EOL, for example https://github.com/access2rohit/incubator-mxnet/blob/u20_ci_update_bp/ci/docker/Dockerfile.build.ubuntu_base_cpu

You can choose to adopt the same approach as on master or just keep the structure as on v1.x. But the goal may still be to avoid using EOL unmaintained base images?