Update default password to environment variable

What does this MR do and why?

In order to fix E2E tests that fail after enabling the password complexity feature on JH, we expect to change the default password used when creating the user to one that meets the password complexity requirements.

So I modified the password method and changed the hardcode to get it from Runtime ENV.

In this file gitlab/qa/qa/resource/user.rb

Before:

      def password
        @password || 'password'
      end

After:

      def password
        @password || Runtime::User.default_password
      end

The relevant context can be viewed from this Issue.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @prajnamas