title: EF7 migration generator is ignoring context parameter
body: <p>I have recently migrated my EF7 project from DotNet Core 1.0.0 to Version 1.1.0, now I have trouble with creating the database migrations.</p>
  <p>In my project I have multiple contexts, the tool dotnet-ef is providing a command line parameter to distinct between these contexts in order to create a separate migration for each context.</p>
  <p>When I try to create the migrations for the context &quot;AuditLogContext&quot; with the command</p>
  <pre><code>dotnet ef migrations add Initial --context 'Some.Namespace.AuditLogContext' --output-dir './Migrations/AuditLogMigrations'
  </code></pre>
  <p>then it fails, telling me</p>
  <blockquote>
  <p>The name 'Initial' is used by an existing migration.</p>
  </blockquote>
  <p>when I'm trying to create the migration with another name, say <em>Initial2</em>, the migration get created but for the wrong context.</p>
  <p>In the folder</p>
  <blockquote>
  <p>./Migrations/AuditLogMigrations</p>
  </blockquote>
  <p>are new files named</p>
  <blockquote>
  <p>20170212122451_Initial2.cs</p>
  <p>20170212122451_Initial2.Designer.cs</p>
  <p>CoreContextModelSnapshot.cs</p>
  </blockquote>
  <p>I expect the file <strong>AuditLogContextModelSnapshot.cs</strong> instead.</p>
  <p>Does anybody have solved this issue before?</p>