Put the password generator into a real .sh file

(cherry picked from commit b2beddb4d35c12537d17454e543363201c578cd8)

util/install/generate-passwords.sh

+#!/usr/bin/env bash
+#
+# Read a list of Ansible variables that should have generated values, and make
+# a new file just like it, with the generated values.
+
+TARGET=${OPENEDX_RELEASE-master}
+wget -q https://raw.githubusercontent.com/edx/configuration/$TARGET/playbooks/sample_vars/passwords.yml -O passwords-template.yml
+
+while IFS= read line; do 
+    # Make a random string. SECRET_KEY's should be longer.
+    length=35
+    if [[ $line == *SECRET_KEY* ]]; then
+        length=100
+    fi
+    REPLACE=$(LC_ALL=C < /dev/urandom tr -dc 'A-Za-z0-9' | head -c$length)
+    # Change "!!null"-to-end-of-line to the password.
+    echo "$line" | sed "s/\!\!null.*/\'$REPLACE\'/"
+done < passwords-template.yml > my-passwords.yml