support proposed/* branches

Provide support for named proposed branches for selection logic

This syncs the get_release_name_from_branch() function from grenade.

Change-Id: I1adabf07cdc3ea6863cd30d8b6454fb40fc20288
diff --git a/functions-common b/functions-common
index 1f90da7..333f31d 100644
--- a/functions-common
+++ b/functions-common
@@ -549,8 +549,7 @@
 # ``get_release_name_from_branch branch-name``
 function get_release_name_from_branch {
     local branch=$1
-
-    if [[ $branch =~ "stable/" ]]; then
+    if [[ $branch =~ "stable/" || $branch =~ "proposed/" ]]; then
         echo ${branch#*/}
     else
         echo "master"