Do not run the pipeline for tags

Currently, if pushing a commit and a corresponding tag, the pipeline will run two times. In the huge majority of cases, both pipelines will have the same result, which seems like a waste of resources, and produces quite some noise, specifically if teams are using notifications sent to IRC or via mail.

This commit tries to improve this situation via disabling the pipeline for tags, by default. However, it's still possible to run the pipeline if tags are pushed, via setting the variable SALSA_CI_ENABLE_PIPELINE_ON_TAGS to one of "1", "yes" or "true".

Closes #39 (closed) Closes #77 (closed)