Bash eval the lines in source-repository scripts

This will allow us to add environment variables to source-repository
scripts. Useful for things like arch-specific download urls.

Change-Id: I4d046825a317dc88db3d7673ad261cedacd1a104

elements/source-repositories/README.md

@@ -36,6 +36,9 @@ fetch/pull section of a review. For example:
     DIB_REPOLOCATION_nova=https://review.openstack.org/openstack/nova
     DIB_REPOREF_nova=refs/changes/72/61972/8
 
+Additionally, the lines in the source-repository scripts are eval'd, so they
+may contain environment variables.
+
 Git sources will be cloned to \<destination\>
 
 Tarballs will be extracted to \<destination\>. Tarballs should contain a

elements/source-repositories/extra-data.d/98-source-repositories

@@ -13,6 +13,8 @@ function get_repos_for_element(){
     local REGEX="^([^ ]+) (git|tar|file|package) ?(/[^ ]+)? ?([^ ]+)? ?([^ ]*)$"
 
     while read line; do
+        # expand variables
+        line=$(eval echo $line)
 
         # ignore blank lines and lines beginning in '#'
         [[ "$line" == \#* ]] || [[ -z "$line" ]] && continue