add option for skipping SSL verification

[vmunishwar]

To fix the issue - #260

[kumaritanushree]

Changes looks good to me. It will be good if we can add test for the same if possible.

[vmunishwar]

@joaopapereira @kumaritanushree - I have tested the changes and they are working fine on a local gitlab server created with a self signed certificate

[joaopapereira]

LGTM

[vmunishwar]

Steps to reproduce locally:

1. Use docker image at https://github.com/danieleagle/gitlab-https-docker/tree/master to start local git lab server. Create a repo.
2. Create self signed certificate using openssl and configure with the server
3. Test vendir sync - It should give an error 'SSL certificate problem: self signed certificate'
4. Tested the fix by adding skipSslVerfication: true. There is no error and repo gets cloned successfully.

[kumaritanushree]

LGTM