Threshold feature CLI changes.

[shdasari]

Threshold feature CLI changes. These changes introduce the following CLI commands for the threshold feature:
CONFIG

1. config priority-group threshold {port_alias} {PGindex} {shared | headroom} {value}
2. config queue threshold {port_alias} {queueindex} {unicast | multicast} {value}
   SHOW
3. show priority-group threshold {shared | headroom}
4. show queue threshold {unicast | multicast}
5. show threshold breaches
   CLEAR
6. sonic-clear priority-group threshold {port_alias} {PGindex} {shared | headroom}
7. sonic-clear priority-group threshold
8. sonic-clear queue threshold {port_alias} {queueindex} {unicast | multicast}
9. sonic-clear queue threshold
10. sonic-clear threshold breach {eventid}

Depends upon:
sonic-net/sonic-buildimage#3501
sonic-net/sonic-swss-common#310
Azure/sonic-tam#12
sonic-net/sonic-swss#1067

[shdasari]

retest this please

[shdasari]

Changes being tracked via #1313.