feature: support update hostname

[wangforthinker]

Signed-off-by: allen.wang allen.wq@alipay.com

Ⅰ. Describe what this PR did

Consider to update container's hostname in a running container, It should join container namespace. So it imports package github.com/opencontainers/runc/libcontainer/nsenter to meet the requirement(about more implementation details refferring to Ⅳ. Describe how to verify it). Otherwise, It provides more register functions which can do more in container namespace.

Ⅱ. Does this pull request fix one issue?

fixes #2179

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

add test TestRunningContainerUpdateHostname and TestStoppedContainerUpdateHostname.

Ⅳ. Describe how to verify it

after doing update hostname, we can exec into container, and run command "hostname" to verify it.

Ⅴ. Special notes for reviews

It adds a package pkg/nsexec, which defines a function NsExec to join container namespace. It provides register functions map to allow do something in container namespace such as update hostname. It registers a reexec command called nsexec, which imports github.com/opencontainers/runc/libcontainer/nsenter to join container namespace by calling C code. Package pkg/nsexec executes new command to call pouchd nsexec.

[allencloud]

I think this pr has updated or added too many codes, so I am afraid we have to add more pr description s to make the demand and implementation clearly. @wangforthinker
Thanks a lot. 😄

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3470d89). Click here to learn what that means.
The diff coverage is 45.55%.

@@            Coverage Diff            @@
##             master    #2230   +/-   ##
=========================================
  Coverage          ?   67.44%           
=========================================
  Files             ?      281           
  Lines             ?    18866           
  Branches          ?        0           
=========================================
  Hits              ?    12724           
  Misses            ?     4626           
  Partials          ?     1516

Flag	Coverage Δ
#criv1alpha1test	31.17% <2.22%> (?)
#criv1alpha2test	35.27% <2.22%> (?)
#integrationtest	40.74% <45.55%> (?)
#unittest	26.59% <4.08%> (?)

Impacted Files	Coverage Δ
pkg/nsexec/nsexec_child_linux.go	0% <0%> (ø)
daemon/mgr/container_update_hostname.go	14.63% <14.63%> (ø)
daemon/mgr/container.go	58.71% <50%> (ø)
pkg/nsexec/nsexec_linux.go	61.53% <61.53%> (ø)

[pouchrobot]

ping @wangforthinker
Conflict happens after merging a previous commit.
Please rebase the branch against master and push it back again. Thanks a lot.