VxGW: backport Phase 1 commits

Including the following from master:

b72cdc6 - A man page for VTEP.
f187556 - Fix a typo in the CLI man page for host command.
35aae3f - Implements VTEP-add CLI.
98dd72a - VTEP binding delete accepts only a single binding.
c4fd94d - Replace VTEP's tunnel-zone-id property with tunnel-zone ObjectRef.

Signed-off-by: Galo Navarro <galo@midokura.com>
Merge pull request #211 from srvaroa/1.5.vxgwbackports

VxGW: backport Phase 1 commits