Allow use of $$ in nuxeo.conf variables

Step to reproduce:

Use Nuxeo 6.0-HF04 or greater. Bug is not reproducible in Nuxeo 6.0 less than HF04

• set a db backend with 'nuxeo$$' as password
• set nuxeo.db.password to 'nuxeo$$' as password in nuxeo.conf
• start the instance -> K.O. (Connection refused)
  It's because of regression introduced by NXP-16318 in 6.0-HF04
• set nuxeo.db.password to 'nuxeo$$$$' as password in nuxeo.conf
• start the instance -> still K.O. (Connection refused)
• set nuxeo.db.password to 'nuxeo$$$$$$' as password in nuxeo.conf
• start the instance -> OK