[sonic-cfggen] Multi-key should be in form of (a,b) instead of 'a|b'

[taoyl-ms]

- What I did
The design of sonic-cfggen is that multi-key should be in the form of ('key1', 'key2') in memory data structure. In this way, the template will be able to utilize one of the keys with simply key[0]. Only when writing into DB, we serialize the keys into format of 'key1|key2'. LOOPBACK_INTERFACE acts as an example.

Please do not use format 'key1|key2' directly in sonic-cfggen as that will cause the -d and -m behaviors to be different. Here's the output in current version:

admin@sonic-7:~$ sonic-cfggen -d -v VLAN_MEMBER
{('Vlan1000', 'Ethernet96'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet40'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet72'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet4'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet80'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet92'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet32'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet84'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet52'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet36'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet56'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet16'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet68'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet64'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet12'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet60'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet8'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet48'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet20'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet88'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet44'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet76'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet24'): {'tagging_mode': 'untagged'}, ('Vlan1000', 'Ethernet28'): {'tagging_mode': 'untagged'}}
admin@sonic-7:~$ sonic-cfggen -m -v VLAN_MEMBER
{'Vlan1000|Ethernet4': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet8': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet32': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet16': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet36': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet12': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet76': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet72': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet44': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet40': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet48': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet80': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet84': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet28': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet88': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet20': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet24': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet64': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet60': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet52': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet56': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet96': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet92': {'tagging_mode': 'untagged'}, 'Vlan1000|Ethernet68': {'tagging_mode': 'untagged'}}
admin@sonic-7:~$ sonic-cfggen -m -v LOOPBACK_INTERFACE
{('Loopback0', '10.1.0.32/32'): {}, ('Loopback0', 'FC00:1::32/128'): {}}
admin@sonic-7:~$ sonic-cfggen -d -v LOOPBACK_INTERFACE 
{('Loopback0', 'FC00:1::32/128'): {}, ('Loopback0', '10.1.0.32/32'): {}}
admin@sonic-7:~$ 

This PR fixes the issue and unify the result from -d and -m into
{('Vlan1000', 'Ethernet96'): {'tagging_mode': 'untagged'} format.