debootstrap_plugin: optionally install TLS Certificate Authority certs

Add an optional tls_ca_certs key which takes a list of paths to TLS Certificate Authority (CA) cert files to install in the image after the debootstrap process has completed. This allows the use of package repositories with HTTPS transports that use TLS certificates issued by private CAs. Note that the CA cert files being installed must have a .crt suffix in order to be used.

This is an alternative approach to fixing the same issue that !124 (closed) resolved by removing the apt-get update command from the debootstrap plugin