Fix multi generic bucket set up

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