Add support for private GitLab repos

[arielvinas]

When using module-install, it assumes the repo origin is from GitHub, e.g:

module-install --module-name 'install-rundeck' --repo 'https://github.com/craftech-io/module-ci' --tag 'v0.0.1'`

This uses GITHUB_OAUTH_TOKEN to authenticate for private GitHub repos. When using GitLab, the common use case is to use deploy tokens.

There should be a variable or automatic way to tell module-installer that the repo is from GitLab, in order to use variables for both the token's username (or if a username isn't provided, should use the default gitlab) and token (or using a variable GITLAB_TOKEN env var). e.g:

using flags:

module-install --module-name 'install-rundeck' --gitlab-token-username username  --gitlab-token <token> --repo 'https://gitlab.com/private-repo' --tag 'v0.0.1'

using env vars:

export GITLAB_TOKEN=<token>
module-install --module-name 'install-rundeck' --repo 'https://gitlab.com/private-repo' --tag 'v0.0.1'`

or

export GITLAB_USERNAME=<token_username>
export GITLAB_TOKEN=<token>
module-install --module-name 'install-rundeck' --repo 'https://gitlab.com/private-repo' --tag 'v0.0.1'`