Don't change the defaul log level during import. Otherwise, the user can...

...'t get the log level he want during theano import.

fix gh-2130
If we do this, we override the user specified level before the import of Theano. I'm not sure what we should do, as this one is used at the info level. I don't like the idea of hidding that output by default and I don't like the idea to change the info level to warn. I'll try to see if we can know if the user set a level for this or not. If so, I'll set the INFO level only if the user didn't provided one.