Create new environment for pre-release branch

[hynguyen2610]

Until now, we've use git process like this:
dev --> stag --> production (master).
We release to stag, and if customers agree on some feature, we cherry-pick it to production. There is a risk, after we cherry pick to production, may be the master branch won't work properly, because there may some things that depend on non-customer-accepted code in stag branch.
So, we will create a buffer environment:
dev --> stag --> pre-production (release) ---> production

• Testers will test on pre-production.
• stag and pre-production will share the database (non-customer-datas, maintained by us)
• Weekly, ready features from dev can be merged to stag, after customer accepted, features will be merged from stag to pre-production, we will see how it work there. If everything is OK, we merge them to production, else refix the pre-production.