build: Change Travis postgres user "root" back to "travis"

build: Change Travis postgres user "root" back to "travis"

Follows-up 5168cb60f8f, in which we moved from Precise vms to Trusty vms.
As a side-effect, the undocumented behaviour of the mysql user "travis"
having create-db rights was no longer. As such, we changed it to "root",
per <https://docs.travis-ci.com/user/database-setup/>.

However, this broke Postgres builds since those should still use
"travis". There is no user named "root" for postgres.

* Add 'dbuser' to the matrix environment.
* Improve inline documentation.

Bug: T75176
Change-Id: I09fc0a1da8737e71b3d2b4b88d72b58c150519c4