bugfix: set canary attributes when initializing balancer

[ElvinEfendi]

What this PR does / why we need it:
These attributes currently are being set only when we sync balancers. But they should also be set when we initialize a balancer for the first time for given backend. Otherwise until the next sync we get errors such as:

2018/11/13 11:00:52 [error] 1434#1434: *14207402 failed to run log_by_lua*: /etc/nginx/lua/balancer.lua:138: attempt to index field 'traffic_shaping_policy' (a nil value)

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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