`/proc` should be remounted even if a nested container set `share_pid_namespace` to true

Currently, if a nested container wants to share the pid namespace of its parent container, we allow the framework to set `LinuxInfo.share_pid_namespace`.

If the nested container does not have its own rootfs (i.e., using the host rootfs), the `/proc` is not re-mounted:
https://github.com/apache/mesos/blob/1.7.x/src/slave/containerizer/mesos/isolators/namespaces/pid.cpp#L120-L126

This is problematic because the nested container will fork host's mount namespace, thus inherit the `/proc` there. As a result, `/proc/<pid>` are still for the host pid namespace. The pid namespace of the parent container might be different than that of the host pid namspace.

As a result, `ps aux` in the nested container will show all process information on the host pid namespace. Although, the pid namespace of the nested container is different than that of the host.