Allow Configuration of how cloning is performed by runner/pipeline

Description

When a pipeline job is run, the first process is to clone the target repo using an HTTP(S) process, which can fail for several reasons (e.g. target remote is missing port during clone):

Running with gitlab-runner 10.4.0 (857480b6)
  on sonarQube (br5ac082)
Using Shell executor...
Running on someServer...
Cloning repository...
Cloning into '/home/gitlab-runner/builds/br5ac082/0/group/proj'...
fatal: repository 'http://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@someServer/group/proj.git/' not found
ERROR: Job failed: exit status 1

Note: external_url was in this case: http://someServer:8888

Proposal

Have the cloning process be configurable for:

• protocol (http, https, ssh)
• authentication (user auth token, gitlab-ci-token, ssh private key)
• port (default to what's specified in external_url)

Links / references

Here is an example issue and workaround employed: https://stackoverflow.com/a/48478314/1080804

Here is a related on-going discussion: https://gitlab.com/gitlab-org/gitlab-ce/issues/22723