Add the option of use only SSO login and register

[melvinsoft]

The PR introduces the ability of redirect the login and register page AKA logistration to an SSO Identity Provider, this is more efficient than using Nginx rules as we currently do.

This is a temporary patch, since we already have a ProdPad idea to implements this upstream first and then start using it.

How to test it:

1. Checkout this branch.
2. Add a new feature flag under APPSEMBLER_FEATURES:

"ENABLE_EXCLUSIVE_SSO_LOGISTRATION": true

3. Add a new Env variable with a url map:

"EXCLUSIVE_SSO_LOGISTRATION_URL_MAP": {
        "register": "http://appsembler.com",
        "login": "http://appsembler.com"
    },

[melvinsoft]

(3) Trinity: Investigates new users without SSO link.

[OmarIthawi]

Looks good Maxi! Thanks!

[OmarIthawi]

@melvinsoft you've used aws.py, but devstack_appsembler.py. Care to unify?

[melvinsoft]

@OmarIthawi Thanks, change addressed.

[OmarIthawi]

Thanks @melvinsoft! Looking forward to get this merged!