Added dockerfile key support

Fixes #486
This commit will add `dockerfilepath` key under Dockerstratergy in buildconfig.
dockerfilepath allow us to use dockerfiles which are named different than `Dockerfile` which are placed in context directory.

for example, for a docker-compose file:

```
version: "2"
services:
    foo:
        build:
            context: "./build"
            dockerfile: "Dockerfile-alternate"
        command: "sleep 3600"
```
Resulting buildconfig will be:

```
apiVersion: v1
kind: BuildConfig
...
...
  strategy:
    dockerStrategy:
      dockerfilePath: Dockerfile-alternate
    type: Docker
...
...
```

pkg/transformer/openshift/openshift.go

@@ -200,7 +200,9 @@ func initBuildConfig(name string, service kobject.ServiceConfig, composeFileDir
 					ContextDir: contextDir,
 				},
 				Strategy: buildapi.BuildStrategy{
-					DockerStrategy: &buildapi.DockerBuildStrategy{},
+					DockerStrategy: &buildapi.DockerBuildStrategy{
+						DockerfilePath: service.Dockerfile,
+					},
 				},
 				Output: buildapi.BuildOutput{
 					To: &kapi.ObjectReference{