build: Use POSIX sh for shell scripts

The scripts didn't really use any bash specific features. Convert
them to POSIX shell scripts, so that the plugins can be built without
requiring bash.

Signed-off-by: Tom Wieczorek <twieczorek@mirantis.com>

scripts/release.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 set -xe
 
 SRC_DIR="${SRC_DIR:-$PWD}"