[cfgmgr]: Integrate MACsecMGR with sonic-buildimage

[Pterosaur]

What I did

1. Fix bug about the return value of get_value, it should return true if it's OK
2. Fix bug about the type of MACsecProfile::priority, the original uint8_t will cause a lexical_convert failure when the priority is a integer.
3. Polish log in get_value.
4. Change the predefined paths for sonic-buildimage integration

Why I did it
I want that the MACsecMgrd can automatically start, so these configuration should be changed to adapt the configuration in sonic-buildimage

How I verified it
Change the /etc/sonic/config_db.json as follow

{
    "PORT": {
        "Ethernet0": {
            ...
            "macsec": "test"
         }
    }
    ...
    "MACSEC_PROFILE": {
        "test": {
            "priority": 64,
            "cipher_suite": "GCM-AES-128",
            "primary_cak": "0123456789ABCDEF0123456789ABCDEF",
            "primary_ckn": "6162636465666768696A6B6C6D6E6F707172737475767778797A303132333435",
            "policy": "security"
        }
    }
}

To execute sudo config reload -y, We should find the following new items were inserted in app_db of redis

127.0.0.1:6379> keys *MAC*
1) "MACSEC_EGRESS_SC_TABLE:Ethernet0:72152375678227538"
2) "MACSEC_PORT_TABLE:Ethernet0"
127.0.0.1:6379> hgetall "MACSEC_EGRESS_SC_TABLE:Ethernet0:72152375678227538"
1) "ssci"
2) ""
3) "encoding_an"
4) "0"
127.0.0.1:6379> hgetall "MACSEC_PORT_TABLE:Ethernet0"
 1) "enable"
 2) "false"
 3) "cipher_suite"
 4) "GCM-AES-128"
 5) "enable_protect"
 6) "true"
 7) "enable_encrypt"
 8) "true"
 9) "enable_replay_protect"
10) "false"
11) "replay_window"
12) "0"

Details if related