Support multiple Ubuntu distros

[wooksong]

Currently, the Ubuntu build checker in TAOS-CI works only for Xenial, which reaches EOL. Moreover, in some cases, it is possible to face different results that are come from differences between the distribution variations.

I think pbuilder and pdebuild have already supported such features so that we could exploit them.

[taos-ci]

cibot: Thank you for posting issue #647. The person in charge will reply soon.

[myungjoo]

Yes.. and it'd be also appropriate to move on to 18.04 (and 20.04 optionally).