services: add CI_DEBUG_SERVICES variable to enable service output to build log

What does this MR do?

• Resolve #2119 as proposed by @ayufan in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/2119#note_24839260
• Adds a CI_DEBUG_SERVICES boolean variable. If this is set, stdout from services containers will be printed in the build log
• In the listing below, waiting for TCP connection to 172.17.0.2:3306 is from the stdout of the mysql service container.

Example output from https://gitlab.com/martinlindhe/ci-test-proj/builds/11674546

Running with gitlab-ci-multi-runner 9.0.0~beta.14.gb03cf20 (b03cf20)
  on b2-linux (2a95689c)
Using Docker executor with image golang:1.8 ...
Starting service mysql:latest ...
Pulling docker image mysql:latest ...
Waiting for services to be up and running...
[runner-2a95689c-project-2808672-concurrent-0-mysql] 0waiting for TCP connection to 172.17.0.2:3306..................
*** WARNING: Service runner-2a95689c-project-2808672-concurrent-0-mysql probably didn't start properly.

service runner-2a95689c-project-2808672-concurrent-0-mysql-wait-for-service did timeout

f2017-03-06T15:29:21.021785753Z error: database is uninitialized and password option is not specified 
�2017-03-06T15:29:21.021807297Z   You need to specify one of MYSQL_ROOT_PASSWORD, MYSQL_ALLOW_EMPTY_PASSWORD and MYSQL_RANDOM_ROOT_PASSWORD

*********

Pulling docker image golang:1.8 ...
BRunning on runner-2a95689c-project-2808672-concurrent-0 via b2...
"Cloning repository...
4Cloning into '/builds/martinlindhe/ci-test-proj'...
/Checking out e3aa38bc as master...
*Skipping Git submodules setup
/$ ln -s /builds /go/src/gitlab.com
\$ cd /go/src/gitlab.com/martinlindhe/ci-test-proj
$ go get -t ./...
>$ go tool vet -composites=false -shadow=true *.go
@$ go test -race $(go list ./... | grep -v /vendor/)
:?   	gitlab.com/martinlindhe/ci-test-proj	[no test files]
Job succeeded

Why was this MR needed?

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

#2119