initial ssh clone support

[nabuskey]

Related to: #385

This adds support for cloning private repositories by SSH.

Currently, when users want to use a private repository, they need to first clone the private repository, then run idpbuilder against the local copy. This allows them to specify a private repository with ssh clone format:

idpbuilder  create --recreate  --package [email protected]:nabuskey/stacks-private//basic/package2

Currently only SSH Key authentication without password is supported.

I am not sure about introducing something like this because:

1. This adds complexity and cannot possibly account for different SSH configurations. e.g. trying all valid ssh keys, accounting for passwords, etc.
2. There's already a valid workaround. Cloning a private repo with way more reliable git clone command.
3. In the future, it may introduce support for http auth which becomes even more complex.