Modify _readEsgfConfig() to work with ESGF 3.0 config file

[William-Hill]

The _readEsgfConfig() function in config.py adds a section header to the esgf.properties to make it conform to the format the ConfigParser Python module expects.

The esgf.properties file in ESGF 3.0 already has a section header. _readEsgfConfig() should check if a section header if already present and skip adding a new header in that case. This should make compatible with 3.0 without breaking 2.x compatibility.