Implement the bundle parameter in multirootca authsign

[jayme-github]

/api/v1/cfssl/authsign now supports the bundle parameter and returns an "optimal" bundle in case it is set in a request (as documented in cfssl/doc/api/endpoint_authsign.txt).

For this to work the rootCA must either be in the systems root store or it might be provided by filename in multirootca.conf, like:

[intermediate1]
private = file:///intermediate/intermediate1-key.pem
certificate =/intermediate/intermediate1.pem
config =/intermediate/intermediate-config.json
ca = /ca/ca.pem