addons: vxlan: single-vxlan: remove local ip cidr before executing iproute2 link add

addons: vxlan: single-vxlan: remove local ip cidr before executing iproute2 link add

When creating a single-vxlan device we were sending local tunnel ip address with ip
network prefix. This patch avoid the following error:

error: cmd '/bin/ip link add dev vxlan0 type vxlan external local 27.0.0.9/32 dstport 4789' failed: returned 1 (Error: any valid address is expected rather than "27.0.0.9/32".
)

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>