Add skip-ssl-validation option to target

- If not specified, certificates are validated.
- Setting is remembered per target URL, even if re-targeted.

Signed-off-by: Madhura Bhave <mbhave@gopivotal.com>

[#74422600] https://www.pivotaltracker.com/story/show/74422600
Bump version to 3.0.0

Signed-off-by: Madhura Bhave <mbhave@gopivotal.com>