Update VPN logging to use new i18n functions

For log messages in neutron/services/vpn and neutron/db/vpn, replace
_() marker functions with log-level-specific marker functions: _LI(),
_LW(), _LE() from oslo.i18n.

Also, remove _() functions for debug log messages as debug level log
messages should not be translated.

Change-Id: I07fcf25bb6344c47e74d6ee23f9bc08e4b560679
Closes-Bug: #1379811
Merge "Update VPN logging to use new i18n functions"