Resolve #6812, limit length of load balancer names

[caesarxuchao]

Use the first 32 bytes of string ("a"+ service.UID) as ELB's name, thus the length of the name is shorter than the requirement of AWS. Adding a prefix "a" is to satisfy GCE's requiements on ELB's name.

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[ghost]

LGTM. Can be merged as soon as Travis or Shippable are green.

[fabioy]

SGTM. Waiting on Green.