Integrated Alerting: User can not receive notifications on email after password update

User Impact:
Users can not receive alert notifications after password change for SMTP.

Steps to reproduce:

1. Go to Settings and enable Integrated Alerting
2. Go to Communication settings and add Slack URL and SMTP settings for receiving emails, e.g
   Server Address: smtp.gmail.com:465
   Username: your email address
   Password: (add some random password)
3. Create some rule that can be violated, add email as a notification channel for the alert
4. Go to Communication Settings and change Password for SMTP server (add the valid one password)
5. Verify if notification is received to the specified channel

Actual result:
alertmanager.yml contains old password for SMTP. Users can not receive emails due to invalid password.
Expected result:
Password for SMTP is updated in alertmanager.yml file. Users can receive notifications to the specified email.