Docker build issues in environments with SSL/TLS inspection

Recently when working on a project where I was building a custom hook on behalf of my customer, I ran into issues firstly with not being able to see the error coming from the build container (as referenced in #250). After finally discovering the error, it turned out to be SSL errors being encountered during the attempt to install dependencies with pip:

`connection error: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:598)`

Upon further inspection, the build environment has TLS inspection enabled and a self-signed certificate is inserted in the chain. The only way to make installations work in this environment outside of using an authenticated pull-through proxy, is by using `--trusted-host xxxxxxxx` flags with pip.

I'd like to open this is a potential feature request where a new flag could be submitted along with the `cfn submit` that would allow for the following flags to be submitted along with the rest of the requisite defaults:

`--trusted-host pypi.org --trusted-host pypi.python.org --trusted-host files.pythonhosted.org`

The flag to be submitted could look something like `cfn submit --trust-self-signed-certs` or `cfn sub --dry-run --trust-self-signed-certs`