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/kobject/kobject.go

@@ -84,6 +84,7 @@ type ServiceConfig struct {
 	Tty           bool                `compose:"tty" bundle:""`
 	MemLimit      yaml.MemStringorInt `compose:"mem_limit" bundle:""`
 	TmpFs         []string            `compose:"tmpfs" bundle:""`
+	Dockerfile    string              `compose:"dockerfile" bundle:""`
 }
 
 // EnvVar holds the environment variable struct of a container