Add Juju Public Key

This commit adds the requirement to provide the Juju public key to N2VC

- Updates integration testing to use public key
- Updates N2VC to accept juju_public_key and ca-cert
- Updated docstring of N2VC constructor

Change-Id: I4cad1f8c39024137a23060000d7502aad56ac644
Signed-off-by: Adam Israel <adam.israel@canonical.com>