joiner: split `ZkFarmJoiner` into `ZkFarmImporter` and `ZkFarmJoiner`

 - `ZkFarmExporter` will take configuration from zookeeper and put it
   into the local filesystem.

 - `ZkFarmImporter` will take configuration from the local filesystem
   and put it into zookeeper.

 - `ZkFarmJoiner` will do both.

The new class `ZkFarmImporter` will be used in the future to add the
ability to import conf to zookeeper without pushing the changes to the
local filesystem. To avoid code duplication, it retains the same FSM
than `ZkFarmJoiner` but the `znode modified` state is unreachable
because we do not listen to the appropriate zookeeper node.
README: add missing configuration about `zkfarmer --join --common`
importer: add unittests for new `ZkFarmImporter`

Those tests are shared with tests for `ZkFarmJoiner` when this is
possible.
zkfarmer: add `zkfarmer import` command

This commands will import local configuration to ZooKeeper but will
not update the local configuration with ZooKeeper content. This is not
the exact reverse of `zkfarmer export` since it is only able to import
one node. This is a limited version of `zkfarmer join` where changes
from ZooKeeper are ignored.

This is useful when you have a large farm of joinder using a common
node to sync configuration.
Merge pull request #36 from vincentbernat/feature/zkimporter

zkfarmer import