Argument "ca_certs" for fast_puller and fast_pusher

[dirkboye]

I'd like to be able to tell puller and pusher which ca_certs to use. Right now the default ones
from "cacerts.txt" of httplib2 are used.

httplib2.Http constructor has argument "ca_certs"

We have an on-premise docker-registry with a local certificate.
Right now I'm appending our cert to the cacerts.txt of httplib2 but I don't like that :-)

[dirkboye]

Just saw that there's an open PR ( #89 ) for this feature. Closing this issue.