[hostcfgd] Fix the state machine during eth0 default route check failure

[vivekrnv]

• The last del command in the following flow will fail because of a bug in MgmtIfaceCfg. During vrf add, it tries to update the default route and if the route is not present, the command will write an error log

• MgmtIfaceCfg is not saving the saving the state in the internal variable. This will cause problem next time and update comes.

  config vrf add mgmt
  config vrf del mgmt
  config vrf add mgmt
  config vrf del mgmt
  

• Added a UT and verified the fix

  Dec 18 03:57:43 sonic hostcfgd[98362]: MgmtIfaceCfg: mgmt vrf state update
  Dec 18 03:57:43 sonic hostcfgd[98362]: Set mgmt vrf state true
  Dec 18 03:57:50 sonic hostcfgd[98362]: MgmtIfaceCfg: Could not delete eth0 route
  Dec 18 03:58:05 sonic hostcfgd[98362]: MgmtIfaceCfg: mgmt vrf state update
  Dec 18 03:58:05 sonic hostcfgd[98362]: Set mgmt vrf state false