Coverity: add the build script used by Travis-CI and Coverity Scan

from https://scan.coverity.com/scripts/travisci_build_coverity_scan.sh
Coverity: customize the build script

- add a 'Verify Coverity Scan run condition' step to avoid multiple runs with
  Travis matrix.
- add a 'Verify Coverity Scan script test mode' step. if true no uploading, to
  avoid reaching the quota. usual processing: false.
- send 'description' as VERSION#SHA (e.g.: 4.6.0-PRE-GIT#c661f8b)
Coverity: update .travis.yml for analysis by pushing in 'coverity_scan' branch