push: show image digest after push succeed
push: show image digest after push succeed

After change :

```
➜  buildah git:(push-fix) ✗ sudo ./buildah push 71c dir:/home/zhouhao/test/buildah
Getting image source signatures
Copying blob sha256:cdb3f9544e4c61d45da1ea44f7d92386639a052c620d1550376f22f5b46981af
 55.73 MiB / 55.73 MiB [====================================================] 1s
Copying blob sha256:7f2cffb520ed710c0871cd07f4fd8041d67fb9b4a74e03d036db55c23a085840
 51.81 MiB / 51.81 MiB [====================================================] 0s
Copying blob sha256:64ef7c2d456f323178e92fd11fd6819c4b913ffedf3d27f8bf12702e6f953d36
 3.50 KiB / 3.50 KiB [======================================================] 0s
Copying config sha256:71c43202b8ac897ff4d048d3b37bdf4eb543ec5c03fd017c3e12c616c6792206
 5.88 KiB / 5.88 KiB [======================================================] 0s
Writing manifest to image destination
Storing signatures
Succefully pushed /home/zhouhao/test/buildah@d963acde7543161ad481fa33d4d03dcbd826826385c8155a5d6b3c89cb297fbb
```

Signed-off-by: Zhou Hao <zhouhao@cn.fujitsu.com>

Closes: #992
Approved by: rhatdan

projectatomic/buildah,nalind/buildah,TomSweeneyRedHat/buildah,ipbabble/buildah,rhatdan/buildah,rhatdan/buildah,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,ipbabble/buildah,projectatomic/buildah,nalind/buildah-1,nalind/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah