Host's repositories management: Update API.md

Define get and create API for repositories collection.
Define get, update, enable and disable API for repository resource.

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>
Host's repositories management: Update REST API

Define Repositories collection and Repository resource according to API.md
Update API.json
Activate auth support to new collection

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>
Host's repositories management: Update backend.

Update model and mockmodel to support backend opertions.
Add new file implementing backend operations. There are two new classes:

1) Repositories (object): Class to represent and operate with repositories
information in Kimchi's perspective. It's agnostic to host;s package management
system, and can execute all operations necessary: add repository, get all
repositories list, get information about one repository, update a repository,
enable and disable a repository and remove a repository. This class will load
in runtime the necessary classes to work with the host's package management:
YumRepo for YUM systems based and AptRepo for APT systems based (support for
this last one will be submited in a close future);

2) YumRepo (object): Class to represent and operate with YUM repositories.
Loaded only on those systems that supports YUM, it's responsible to connect,
collect and provide information of YUM repositories in the system. Also it's
responsible to create/delete the files in disk to maintain the repositories in
system after disconnection.

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>
Host's repositories management: Update Makefile

Update Makefile files to provide repositories support.

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>
Host's repositories management: Update test-cases.

Added unit tests into test_model.py and test_rest.py

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>
Host's repositories management: Update error exception messages.

Added new error exception messages for host's repositories support.

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>