tf-a-lts-docs-watcher.sh: Convert Git tag to RTD slug once

Make RTD-related functions to accept a slug, and convert incoming Git tag
to RTD slug once at the beginning and then pass around.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ie5b8424d0acf75d978440ad6b3e532e032f14feb
1 file changed