Fix configuration parser crash

The configuration file parser is crashing on tin with this error:
    22:21:43 DEBUG    - Unhandled error:
    Traceback (most recent call last):
    File "/srv/scap/scap/main.py", line 142, in scap
        cfg = load_config(args)
    File "/srv/scap/scap/main.py", line 36, in load_config
        cfg = config.load(args.conf_file, defines)
    File "/srv/scap/scap/config.py", line 70, in load
        config.update(parser.items(section))
    File "/usr/lib/python2.7/ConfigParser.py", line 655, in items
        for option in options]
    File "/usr/lib/python2.7/ConfigParser.py", line 691, in _interpolate
        self._interpolate_some(option, L, rawval, section, vars, 1)
    File "/usr/lib/python2.7/ConfigParser.py", line 700, in _interpolate_some
        p = rest.find("%")
    AttributeError: 'int' object has no attribute 'find'

Disabling configuration item interpolation seems to fix this. An
alternate way to fix the problem would be to alway ensure that all
values in the configuration file are quoted. I think until we find a use
case for interpolation this is the safer course of action.

Change-Id: I2ab8187b19c0259fa556be463bb4a8ec71c15a29