Changes

jenkins-common: Treat -asan builds the same as -master

This fixes the jenkins test failures seen in ttcn3-stp-test-asan. TTCN3
tries to connect to stp via TCP, but since the jenkins.sh script doesn't
consider this a build from master the corresponding config is not added
to osmo-stp.cfg.

Also change the function names from image_suffix_is_* to osmo_repo_is_*
to better reflect what they are doing (instead of how).

Related: OS#6486
Change-Id: Ibf93880345f38668f8dc812d1523b985d368dfcc