Check that key exists in couchbase_download_links before dereferencing
Ensure pkgconfig is present

This is a prerequisite of couchbase-server, and while most base boxes have it installed by default the libvirt one doesn't...
Specify box name for centOS64

This is needed for libvirt
Allow #CPUs and RAM to be configured via env vars
Merge pull request #3 from couchbaselabs/libvirt

Libvirt support and more ENV variables