move IsInstalled and apt-fast

images/linux/scripts/helpers/install.sh

@@ -33,4 +33,13 @@ download_with_retries() {
 
     echo "Could not download $URL"
     return 1
+}
+
+## Use dpkg to figure out if a package has already been installed
+## Example use:
+## if ! IsInstalled packageName; then
+##     echo "packageName is not installed!"
+## fi
+function IsInstalled {
+    dpkg -S $1 &> /dev/null
 }