glusterfs: update for new API

[Meoop]

This PR does the following two parts of work：

• Adapt to the newer interfaces to compile against release-6 apis. In order to work with old versions of GlusterFS, it is required to pass build-tags (glusterfs_legacy_api) to the go command line. For example, Compile with < gluster6:

  go build -tags glusterfs_legacy_api ....
  go test -tags glusterfs_legacy_api ....
  

• Update travis testing, complete the test in the container to achieve the purpose of testing both the legacy APIs and the enhanced APIs.

Reference PR:

• QEMU PR: https://patchwork.kernel.org/patch/10838017/
• FIO PR: https://github.com/axboe/fio/commit/ce4d13ca162df4127ec3b5911553802c53396705

Fix issue: #37