Add better support for disabling SSL or using custom SSL certs for project import

Original discussion here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3892#note_5087741

Right now if you attempt to import from GitHub Enterprise or some other site that uses self-signed certs, you have to:

1. Set verify_ssl to false
2. Disable SSL verification in git via git config --global http.sslVerify false

Ideally, we want to provide users:

1. The ability to specify the certs to verify against
2. If they disable SSL verification with verify_ssl, we run GIT_SSL_NO_VERIFY=true git clone XXX instead of setting it globally