fix parsing parameter

[wuciawe]

It resolves #126 and related parsing problems, #112 and #119 .

[wuciawe]

I find that this is related with #112 again. And I think maybe it's necessary to add more tests, and unify the access to properties. All the accesses should be limited with the access provided by com.stratio.datasource.util.Config.

[wuciawe]

I realized that it requires more changes. I'm working on this problem.

[wuciawe]

I hope this time there will be no more paring problems, and I try my best to keep the original structure. In the com.stratio.datasource.mongodb.client.MongodbClientFactoryTest, I changed the way to init configurations since I think it's good to init configuration only from MongodbConfigBuilder and the old short cut way is imcompatitable with the new change. @pfcoperez plz check this pull request, it passes the current project tests, but I'm not very sure it covers all the problems, maybe need more tests and documentation.