Look up the value for an option.

def _option(value):
    '''
    Look up the value for an option.
    '''
    if value in __opts__:
        return __opts__[value]
    master_opts = __pillar__.get('master', {})
    if value in master_opts:
        return master_opts[value]
    if value in __pillar__:
        return __pillar__[value]