Support for defining addons_path in several lines

Impacted versions:

* 8.0

Steps to reproduce:

1. Define addons_path using ConfigParser syntax for several lines. For example
```bash
root_path = /var/openerp/odoo
addons_path: /usr/lib/python2.7/dist-packages/openerp/addons,
             %(root_path)s/addons/,
             %(root_path)s/extras/
```
2. openerp/tools/config.py will parse adddons folder starting with '/\n'

Current behavior:
```bash
openerp: addons paths: ['/var/openerp/.local/share/Odoo/addons/8.0', u'/var/openerp/master/addons', u'/\n/var/openerp/master/partner-contact', u'/\n/var/openerp/master/bank-statement-reconcile', u'/\n/var/openerp/master/account-payment', u'/\n/var/openerp/master/l10n-spain', u'/\n/var/openerp/master/extras', '/var/openerp/master/openerp/addons']
```

Expected behavior:
```bash
openerp: addons paths: ['/var/openerp/.local/share/Odoo/addons/8.0', u'/var/openerp/master/addons', u'/var/openerp/master/partner-contact', u'/var/openerp/master/bank-statement-reconcile', u'/var/openerp/master/account-payment', u'/var/openerp/master/l10n-spain', u'/var/openerp/master/extras', '/var/openerp/master/openerp/addons']
```