[passwordstore] Use builtin _random_password function instead of pwgen (#25843)

* [password] _random_password -> random_password and moved to util/encrypt.py
* [passwordstore] Use built-in random_password instead of pwgen utility
* [passwordstore] Add integration tests

test/utils/docker/opensuse42.3/Dockerfile

@@ -19,6 +19,7 @@ RUN zypper --non-interactive --gpg-auto-import-keys refresh && \
     mariadb \
     mercurial \
     openssh \
+    password-store \
     postgresql-server \
     python-cryptography \
     python-devel \