Merge pull request #5147 from opf/Update-supported-os

Drop CentOS6/RHEL 6 and add Ubuntu 16.04

doc/operation_guides/system_requirements.md

@@ -15,11 +15,11 @@ provide any official support for them.
 
 | Distribution (64 bits only)     | Identifier   | init system |
 | :------------------------------ | :----------- | :---------- |
-| Ubuntu 14.04 Trusty             | ubuntu-14.04 | upstart     |
+| Ubuntu 14.04 Trusty Tahr        | ubuntu-14.04 | upstart     |
+| Ubuntu 16.04 Xenial Xerus       | ubuntu-14.04 | upstart     |
 | Debian 8 Jessie                 | debian-8     | systemd     |
 | Debian 7 Wheezy                 | debian-7     | sysvinit    |
 | CentOS/RHEL 7.x                 | centos-7     | systemd     |
-| CentOS/RHEL 6.x                 | centos-6     | upstart     |
 | Fedora 20                       | fedora-20    | sysvinit    |
 | Suse Linux Enterprise Server 12 | sles-12      | sysvinit    |