The previously documented way to specify multiple generic buckets
doesn't work because vagrant-cachier can't enable a bucket type more
than once.
Here we generalize the configs hash that the generic bucket gets making
it possible to specify multiple buckets with a single hash. The
documentation is changed accordingly.
Note that we keep it backwards compatible for single generic bucket
specification.
Issue: https://github.com/fgrehm/vagrant-cachier/issues/99
8ac8d7f6f6