password needs to be type str.

In 1.9 this was the default.  In 2.0.0 and 2.0.1 this doesn't get set.
2.1.0 will fix the default but we should fix this now.

lib/ansible/modules/database/mysql/mysql_user.py

@@ -476,7 +476,7 @@ def main():
             login_port=dict(default=3306, type='int'),
             login_unix_socket=dict(default=None),
             user=dict(required=True, aliases=['name']),
-            password=dict(default=None, no_log=True),
+            password=dict(default=None, no_log=True, type='str'),
             encrypted=dict(default=False, type='bool'),
             host=dict(default="localhost"),
             host_all=dict(type="bool", default="no"),