[security] make it easier to redefine Alpha/Gamma (#7036)
[security] make it easier to redefine Alpha/Gamma (#7036)

* [security] make it easier to redefine Alpha/Gamma

While talking about some security aspect and as to how you'd alter Alpha
or Gamma role in a specific environment, I realized that these
module-scoped constants would be much more useful as class attributes.

This way, someone can override these sets in their security manager
to alter base roles.

* fix

* flake8

apache/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,airbnb/superset,mistercrunch/panoramix,apache/incubator-superset,airbnb/superset,airbnb/caravel,airbnb/caravel,zhouyao1994/incubator-superset,apache/incubator-superset,airbnb/superset,apache/incubator-superset,mistercrunch/panoramix,mistercrunch/panoramix,airbnb/caravel,mistercrunch/panoramix,airbnb/superset,zhouyao1994/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset