Adds the tenant_filter_file to the config file

tenant_filter_file is the path to a file that contains a list of tenant
ids to migrate. If missing, swsync will migrate all the tenants.
Prevent catching of IOError in case tenant_filter_list is defined

This means the only way to do a full sync is to omit the field from the config
file.
Fixes flake8 issues
Fix test failing when missing a config field
Replaces IDs with tenant_name in filter list
Unit tests coverage for get_target_tenant_filter
Fixes tenant_name vs name confusion
Adds tests for the tenant filter feature
Updates README and other documentation fixes
Merge pull request #62 from enovance/tenant_filter

Adds the tenant_filter_file to the config file