Deploy edx-themes:master instead of edx-themes:release

We want the release pipelines to deploy edx-themes:master to avoid
the additional step of merging edx-themes:master to edx-themes:release
in order to deploy changes to edx-themes.

WL-1018

playbooks/roles/edx_themes/defaults/main.yml

@@ -23,7 +23,7 @@ THEMES_GIT_PROTOCOL: "{{ COMMON_GIT_PROTOCOL }}"
 THEMES_GIT_MIRROR: "{{ COMMON_GIT_MIRROR }}"
 THEMES_GIT_PATH: "{{ COMMON_GIT_PATH }}"
 THEMES_REPO: "sample-themes.git"
-THEMES_VERSION: "release"
+THEMES_VERSION: "master"
 
 
 THEMES_REPOS: