[swss]: update swss docker to stretch

[lguohan]

sonic-swss update:

• 60a8a0d 2019-03-28 | [orchagent]: Cast enum class variable to int ([sonic-mgmt]: move sonic-mgmt docker build into sonic-slave #819) (HEAD, origin/master, origin/HEAD) [Shuotian Cheng]
• 3dd37a4 2019-03-26 | [vnetorch]: Add VNET/tunnel/route removal flows for Bitmap VNET implementation ([platform/Seastone] Add a new supported platform, Celestica Seastone-DX010 #816) [Volodymyr Samotiy]
• a937f92 2019-03-22 | [VS]: fix occasional test_fdb_notifications vs test failure ([device/Seastone]: Add / update a new supported device and platform, Celestica Seastone-DX010 #813) [Jipan Yang]
• ea54825 2019-03-21 | [portsorch] Fix inconsistent return value in bindAclTable ([teamd]: Fix Jinja2 template for calculating min_ports #791) [yorke]
• 5984e3a 2019-03-07 | Fix orchagent SEGV when PortConfigDone not set ([Broadcom]: Update SAI to 2.1.5.1-16 #803) [Ramesh Santhanakrishnan]

- What I did

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[stcheng]

vnetorch.cpp: In destructor 'virtual VNetBitmapObject::~VNetBitmapObject()':
vnetorch.cpp:1102:55: error: throw will always call terminate() [-Werror=terminate]
         throw std::runtime_error("VNET removal failed");
                                                       ^
vnetorch.cpp:1102:55: note: in C++11 destructors default to noexcept
vnetorch.cpp:1107:55: error: throw will always call terminate() [-Werror=terminate]
         throw std::runtime_error("VNET removal failed");
                                                       ^
vnetorch.cpp:1107:55: note: in C++11 destructors default to noexcept