Make IAM gRPC transport a configuration item

Harsha Muralidharrequested to merge
hm-iam-grpc-transport-config into master

Why this change

Currently, GitLab webservice decides whether to encrypt the connection to IAM service based on Rails.env (development mode check). This change allows us to configure grpc:// communication without the tls:// prefix by configuring it in config/gitlab.yml.

What happens now

The configured host may carry a scheme, and that scheme decides the transport:

grpc.host: iam.example.com          ->  tls://iam.example.com:5005   encrypted (unchanged)
grpc.host: tls://iam.example.com    ->  tls://iam.example.com:5005   encrypted (unchanged)
grpc.host: grpc://iam.example.com   ->  grpc://iam.example.com:5005  plain text, logged

Relates to: #619690 (closed)

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