refactor repetative if statements in the scanner.py

[sudiptob2]

related to #153

Idea on refactoring repetitive if statements in the scanner.py

# crawl_name: client_name map
# crawl_name indicates the string depending on which crawl factory will work
# client_name indicates the string depending on which current client factory works
# crawl_name in the client_factory will be chosen in such a way that it matches with the keys in the config file

crawl_client_map = {
  'compute_instances': 'compute',
  'compute_images': 'compute',
  'machine_images': 'compute',
  'compute_disks': 'compute',
  'static_ips': 'compute',
  'app_services': 'appengine',
  'storage_buckets': 'storage',
  'managed_zones': 'managed_zones',
  # and so on
}

# replacing the repetitive if statements

for crawler_name, client_name in crawl_client_map.items():
  if is_set(scan_config, crawler_name):
    CrawlFactory.create_crawler(crawler_name).crawl(
      project_id,
      ClientFactory.get_client(client_name).get_service(credentials),
    )