* Add state option to git_config module
State present/absent option works like --set/--unset option for 'git config'.
* Change git_config to avoid useless parameter passed to git command
When unsetting value, command was : git config --unset foo ''.
Now command is : git config --unset foo.
* Add some integration tests to git_config module
* Add missing aliases file
* Change set up method
Using git command seems to cause troubles on some OS : changing config
by changing '.gitconfig' file.
* Remove some distros from tests pool
Git is not installed or is out of date on these distros.
* Fix aliases to skip tests on centos6
* Refactor tests of the git_config module
* Add use case when state=absent and value is defined
