[config] Add commands for adding/removing syslog servers

[tsvanduyn]

- What I did
I've added config commands to add/del syslog servers from the device.

- How I did it

admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ redis-cli -n 4 keys "*" | grep syslog  -i
SYSLOG_SERVER|100.127.20.21
SYSLOG_SERVER|10.3.145.8
SYSLOG_SERVER|4.4.4.4
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ sudo config syslog add 5.5.5.5
Syslog server 5.5.5.5 added to configuration
Restarting rsyslog-config service...
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ redis-cli -n 4 keys "*" | grep syslog  -i
SYSLOG_SERVER|100.127.20.21
SYSLOG_SERVER|10.3.145.8
SYSLOG_SERVER|4.4.4.4
SYSLOG_SERVER|5.5.5.5
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ sudo config syslog del 5.5.5.5
Syslog server 5.5.5.5 removed from configuration
Restarting rsyslog-config service...
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ redis-cli -n 4 keys "*" | grep syslog  -i
SYSLOG_SERVER|100.127.20.21
SYSLOG_SERVER|10.3.145.8
SYSLOG_SERVER|4.4.4.4
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ 

- How to verify it

There are two ways: 
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ redis-cli -n 4 keys "*" | grep syslog  -i
SYSLOG_SERVER|100.127.20.21
SYSLOG_SERVER|10.3.145.8
SYSLOG_SERVER|4.4.4.4

Or 
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ show run all | grep -i syslog -A4
    "SYSLOG_SERVER": {
        "4.4.4.4": {}, 
        "10.3.145.8": {}, 
        "100.127.20.21": {}
    }, 
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ sudo config syslog add 5.5.5.5
Syslog server 5.5.5.5 added to configuration
Restarting rsyslog-config service...
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ show run all | grep -i syslog -A4
    "SYSLOG_SERVER": {
        "4.4.4.4": {}, 
        "5.5.5.5": {}, 
        "10.3.145.8": {}, 
        "100.127.20.21": {}
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ 

- Previous command output (if the output of a command-line utility has changed)
There was no previous command to add or remove syslog servers.
- New command output (if the output of a command-line utility has changed)

config add syslog command:
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ sudo config syslog add 5.5.5.5
Syslog server 5.5.5.5 added to configuration
Restarting rsyslog-config service...
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$

config del syslog command: 
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ sudo config syslog del 5.5.5.5
Syslog server 5.5.5.5 removed from configuration
Restarting rsyslog-config service...
admin@str-s6000-acs-11:/usr/lib/python2.7/dist-packages/config$ 

-->

[jleveque]

@tsvanduyn: Can you please also update the Command Reference Guide (https://github.com/Azure/sonic-utilities/blob/master/doc/Command-Reference.md) to add these commands in this PR? I forgot to ask this of you with your previous DHCCP Relay destination change. I would like you to open a separate PR to add those commands later, also.

[jleveque]

Retest this please

[stephenxs]

@jleveque @tsvanduyn
It seems that the function name "syslog" conflicts with the module "syslog", causing functions calling logxxx failed, like "config load_minigraph".
Is it possible to rename it, like "logserver"?

[nazariig]

@stephenxs / @jleveque the ticket: #635