base_url is not properly validated in the config file

## Summary
base_url validates fine even if there's a typo, e.g missing `:`

## Steps to reproduce
[secli]
base_url = "https//pulp-1.example.com"

## Expected behavior
`pulp config validate --strict` should fail

## Pulp and pulp-cli version info
pulp3 command line interface, version 0.14.0


