refactored cluster: nodes are now added by the client through the method `add_node` or `add_nodes`. The cluster doesn't add nodes anymore. Also the cycle dependency between cluster and configuration is solved.
refactoring min_nodes: the min_nodes property is now a parameter to the cluster.start method.
Also add 'ansible_ssh_user' to the inventory file
Merge branch 'refactoring' of github.com:gc3-uzh-ch/elasticluster into refactoring
Add documentation for abstract constructor of `AbstractCloudProvider`
class.
Call `SetupProvider` constructor with keyword arguments only.
refactoring cluster repository: to make elasticluster better extensible the `ClusterStorage` is moved to the repository module and implements the `AbstractClusterRepository` contract. The old json implementation is replaced with pickle to store all relevant information of a cluster into the storage file.
Merge branch 'refactoring' of https://github.com/gc3-uzh-ch/elasticluster into refactoring
refactored cluster: nodes are now added by the client through the method `add_node` or `add_nodes`. The cluster doesn't add nodes anymore. Also the cycle dependency between cluster and configuration is solved.
refactoring min_nodes: the min_nodes property is now a parameter to the cluster.start method.
Add documentation for abstract constructor of `AbstractCloudProvider`
class.
Call `SetupProvider` constructor with keyword arguments only.
refactoring cluster repository: to make elasticluster better extensible the `ClusterStorage` is moved to the repository module and implements the `AbstractClusterRepository` contract. The old json implementation is replaced with pickle to store all relevant information of a cluster into the storage file.
Merge branch 'refactoring' of https://github.com/gc3-uzh-ch/elasticluster into refactoring

Conflicts:
	elasticluster/cluster.py
	elasticluster/providers/ansible_provider.py

merge d8739ec from master with refactoring branch
refactoring ansible provider: remove configuration file relevant parsing from the ansible provider to the conf module. Refactoring of constructor parameters to make it clear what is needed and what not.
docstring update for all classes provided by the api in the future
API relevant imports added
bugfix pprint property redo
bugfix encoding error
removed template parameter from cluster.