Add support for using SSH

Add support for using SSH

This patch changes "create_config.py" script interface and its
behaviour:

* by default anonymous Gerrit is used for repository cloning and code
  URLS,
* external git mirror for code URLs might be provided but there is no
  default one,
* SSH credentials might be provided for repository cloning.

Cloning repository using SSH is required due to current ONAP
infrastructure HTTPS bandwidth constraints.

User wishing to use this setup should provide two environmental
variables:

1) API_USER - Gerrit username for cloning repositories via SSH
2) API_KEY - Gerrit API key (User menu -> Settings -> HTTP Credentials)

Be aware that a new RSA key will be generated and added to the user's
Gerrit account for purposes of Hound code search operation inside a VM.
Do remember to delete it (User menu -> Settings -> SSH keys) after
evaluating this environment.

Issue-ID: ONAPARC-540
Change-Id: I3e98dfa8582322d5b3bbbbf377d748faea0da57b
Signed-off-by: Pawel Wieczorek <p.wieczorek2@samsung.com>