Changes to locate site-config

With this commit the system for locating the site config file is as
follows:

 1. During the configure process cmake will create a file site_config.py
    in the build/python/ert_gui directory, the content of this file will
    be:

        config_file = "${SITE_CONFIG_FILE}"

     where ${SITE_CONFIG_FILE} is the variable set during the configure
     step. The generated file will be installed alongside the gert_main
     module.

 2. The gert_main module tries to import the site_config module and uses
    the config_file setting from there.

 3. If the ERT_SITE_CONFIG environment variable has been set that will
    take presedence.

With these changes it is not necessary for the frontend script to set
the ERT_SITE_CONFIG variable.
Merge pull request #586 from joakim-hove/site-config-build-fix

Changes to locate site-config