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* Debian tag should be derived from debian version
* Python tag will be the same for different debian revisions

Refs #3520

Fix value of build bode when "DEVFLOW_BUIILD_MODE" is not set.
Default value is "release" for master, and "snapshot" for all other
branches.

Try to get "debian-*" branches from origin, before falling
back to debian branch.

Find the correct debian revision, by looking the debian tags. Also, move
some functions to new 'utils' module.