api.js refactor using classes (#250)

Refactor api.js into an api module where endpoint specific models can be imported and used in components.

src/api/mixins/InstanceGroups.mixin.js

@@ -0,0 +1,15 @@
+const InstanceGroupsMixin = (parent) => class extends parent {
+  readInstanceGroups (resourceId, params = {}) {
+    return this.http.get(`${this.baseUrl}${resourceId}/instance_groups/`, { params });
+  }
+
+  associateInstanceGroup (resourceId, instanceGroupId) {
+    return this.http.post(`${this.baseUrl}${resourceId}/instance_groups/`, { id: instanceGroupId });
+  }
+
+  disassociateInstanceGroup (resourceId, instanceGroupId) {
+    return this.http.post(`${this.baseUrl}${resourceId}/instance_groups/`, { id: instanceGroupId, disassociate: true });
+  }
+};
+
+export default InstanceGroupsMixin;