travis: upgrade to bionic, clang-9, improve readability

Changes, partially to reduce build failures from external dependencies:
 - Upgrade Ubuntu and drop unnecessary third-party repos.
 - Properly clone apt config to ensure retries.
 - Upgrade to clang-9 from the standard repos.
 - Use Ubuntu 20.04 focal for the libssh build, use of ssh_get_publickey
   fails on -Werror=deprecated-declarations in Ubuntu 18.04. Do not use
   focal everywhere yet since Travis CI has not documented this option.
   In focal, python-impacket (Py2.7) has been removed, leaving only
   python3-impacket. Since it is only needed for SMB tests and not SSH,
   skip it for the libssh job since it might need more work.
 - apt: Remove gcc-8 and libstdc++-8-dev, already installed via g++-8.

Non-functional cleanups:
 - Simplify test matrix, drop redundant os and compiler keys.
 - Deprecation fixes: remove sudo, rename matrix -> jobs.
 - Every job has an 'env' key, put this key first in a list item.

Closes #5370

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