Remove exception throwing when initializing missing loopback interface (#90)

### Description of PR
Summary:
Fixes # (issue)
This PR is to avoid throwing exception when linkmgrd initializing without CONFIG DB loopback interface entry. 

sign-off: Jing Zhang [email protected] 

### Type of change
- [x] Bug fix

### Approach
#### What is the motivation for this PR?
Avoid unnecessary process exits. 

#### How did you do it?
Log a fatal message instead of exiting the process. 

#### How did you verify/test it?
Tested on dual testbeds. Met all expected behavior below:
1. Remove loopback interface live (when linkmgrd is running):   
    No linkmgrd crash. `show mux status` reported `unhealthy` as no IP interface to receive heartbeat packets.
2. Start mux service when missing loopback interface:  
    No linkmgrd crash. Saw fatal log below in syslog. `show mux status` report `unhealthy`.
3. Add back loopback interface CONFIG DB entry live:  
    `show mux status` reported `healthy`.

Author: zjswhhh

src/DbInterface.cpp

@@ -577,7 +577,7 @@ void DbInterface::processLoopback2InterfaceInfo(std::vector<std::string> &loopba
     }
 
     if (!loopback2IPv4Found) {
-        throw MUX_ERROR(ConfigNotFound, "Loopback2 IPv4 address missing");
+        MUXLOGFATAL(boost::format("Config not found: Loopback2 IPv4 address missing, using default value %s ") % mMuxManagerPtr->getLoopbackIpv4Address().to_string());
     }
 }
 

src/MuxManager.h

@@ -193,6 +193,15 @@ class MuxManager
     */
     void setUseWellKnownMacActiveActive(bool useWellKnownMac);
 
+    /**
+     * @method getLoopbackIpv4Address
+     * 
+     * @brief getter for loop back ipv4 address 
+     * 
+     * @return IPv4 address
+     */
+    inline boost::asio::ip::address getLoopbackIpv4Address() {return mMuxConfig.getLoopbackIpv4Address();};
+
     /**
     *@method initialize
     *

test/MuxManagerTest.cpp

@@ -440,6 +440,7 @@ TEST_F(MuxManagerTest, Loopback2Address)
     processLoopback2InterfaceInfo(loopbackIntfs);
 
     EXPECT_TRUE(getLoopbackIpv4Address(port).to_string() == ipAddress);
+    EXPECT_TRUE(mMuxManagerPtr->getLoopbackIpv4Address() == getLoopbackIpv4Address(port));
 }
 
 TEST_F(MuxManagerTest, Loopback2AddressException)
@@ -448,13 +449,14 @@ TEST_F(MuxManagerTest, Loopback2AddressException)
 
     createPort(port);
 
+    std::string defaultAddress = "10.212.64.0";
     std::vector<std::string> loopbackIntfs = {
         "Loopback2|2603:10e1:100:d::1/128",
         "Loopback2|250.260.270.280/32",
         "Loopback2"
     };
 
-    EXPECT_THROW(processLoopback2InterfaceInfo(loopbackIntfs), common::ConfigNotFoundException);
+    EXPECT_TRUE(getLoopbackIpv4Address(port).to_string() == defaultAddress);
 }
 
 TEST_F(MuxManagerTest, ToRMacAddress)