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{

pkg/transformer/openshift/openshift_test.go

@@ -290,7 +290,8 @@ func TestInitBuildConfig(t *testing.T) {
 	repo := "https://git.test.com/org/repo"
 	branch := "somebranch"
 	sc := kobject.ServiceConfig{
-		Build: "./build",
+		Build:      "./build",
+		Dockerfile: "Dockerfile-alternate",
 	}
 	bc, err := initBuildConfig(serviceName, sc, composeFileDir, repo, branch)
 	if err != nil {
@@ -305,6 +306,7 @@ func TestInitBuildConfig(t *testing.T) {
 		"Assert buildconfig source git Ref":     {bc.Spec.CommonSpec.Source.Git.Ref, branch},
 		"Assert buildconfig source context dir": {bc.Spec.CommonSpec.Source.ContextDir, "a/build"},
 		"Assert buildconfig output name":        {bc.Spec.CommonSpec.Output.To.Name, serviceName + ":latest"},
+		"Assert buildconfig dockerfilepath":     {bc.Spec.CommonSpec.Strategy.DockerStrategy.DockerfilePath, "Dockerfile-alternate"},
 	}
 
 	for name, test := range testCases {