Implement smarter auth source identification for AnsibleTower

[JacobCallahan]

@rplevka had a great suggestion in #177 for a better way to determine which auth sources an AAP instance supports.

an unauthenticated request to <aap_server>/api/ resource
returns the following info:

{
"description": "AWX REST API",
"current_version": "/api/v2/",
"available_versions": {
"v2": "/api/v2/"
},
"oauth2": "/api/o/",
"custom_logo": "",
"custom_login_info": "",
"login_redirect_override": ""
}
Wouldn't it be better if broker fetched the custom_login_info value and display it as warning if set?
This way, we could keep the login-specifics to the individual provider instances instead of hardocding stuff.

We should evaluate this and see if we can use this instead of the current version implemented in that PR.

[JacobCallahan]

this could possibly be attempted by creating an instance of the unauthenticated API and running api.get() to get the custom_login_info. Though we'd need to have a standard telling us what's supported and what isn't.

In [12]: api.get()
Out[12]: 
{
    "description": "AWX REST API",
    "current_version": "/api/v2/",
    "available_versions": {
        "v2": "/api/v2/"
    },
    "oauth2": "/api/o/",
    "custom_logo": "",
    "custom_login_info": "",
    "login_redirect_override": ""
}