Improve cookie setup for RPMs

Improve cookie setup for RPMs

Previously, with an embedded prompt, it was hard to automatically
provision nodes.

Avoid the interactive TTY prompt by setting a random 48 character (285
bits of entropy) cookie from /dev/urandom instead. This should help
automating standalone setups.

Improve clustered setups by allowing users to specify the cookie as an
environment variable. In this way the cookie may be automaticaly
provisioned on all the nodes of the cluster during the initial
install.

Fixes https://github.com/apache/couchdb-pkg/issues/94