sonic-utilities submodule update

9a88cb6 [sonic_installer] dont fail package migration (#1591)
615e531 [show][config] Add new snmp commands (#1347)
fff4051 Fixing serial number read to get from DB if it is populated (#1580)
be974bf [neighbor_advertiser] Use existing tunnel if present for creating tunnel mappings (#1589)
9492eab Use swsscommon instead of swsssdk (#1510)
0f4988b Add pg-drop script to sonic filesystem (#1583)
cbe2159 [vnet] Add "vnet_route_check" script (#1300)
9120766 Relax the install_requires, no need to exact version as long as there are no broken changes with future versions (#1530)
2e09b22 Handle the new db version which mellanox_buffer_migrator isn't interested (#1566)

Author: dgsudharsan