Read relevant DbContextOptions from IConfiguration

[divega]

We should make the following pattern work:

{
    "EntityFramework": {
        "MyContext": { 
            "ConnectionString": 
"Server=(localdb)\\mssqllocaldb;Database=MyDatabase;Trusted_Connection=True;MultipleActiveResultSets=true"
        }
    }
}

// Startup.ConfigureServices:
services.AddEntityFramework(configuration)
    .AddSqlServer()
    .AddDbContext<MyContext>();

// MyDbContext.OnConfiguring:
modelBuilder.UseSqlServer(); //Matches "EntityFramework:MyContext:ConnectionString" by convention

Lacking the ability to read the connection string from configuration you are forced to read it explicitly in Startup.cs, e.g. with configuration.Get("EntityFramework:MyContext:ConnectionString").

Implementation details:

1. AddEntityFramework only captures the IConfiguration argument and flows it through the EntityServicesBuilder to AddDbContext.
2. AddDbContext will stick an IConfigureOptions<DbContextOptions> into DI with configuration Order (precedence) which will read all sub keys under "EntityFramework:MyContext" into a property bag into the DbContextOptions instance.

[divega]

@rowanmiller, @ajcvickers this is the work item we talked about earlier. I assigned it to @emilcicos for beta.

[rowanmiller]

Just adding a note to capture the idea that ultimately we could have the config section also specify the provider to use so that modelBuilder.UseSqlServer(); is not required in code... but we're not planning to work on that at the moment.

commit a4b0244
Current code reads the configuration but only uses ConnectionString (ConnectionStringKey). The rest need to be done after beta.