Wrong key router.interface reported by ceilometer

Ceilometer with MongoDB doesn't accept the key with dot.

The dot in 'router.interface' should be replaced with '_',
which is also the standard implementation of other keys in neutron.

Change-Id: If7fcf7658db9ee2d7d1248bb0b016b4389c32c77
Related-Bug: #1310460
(cherry picked from commit 7811c8a8ffce78dc14815203858854c450f1c8f6)
Merge "Wrong key router.interface reported by ceilometer" into stable/icehouse