potential fix for attaching router with external network gateway

potential fix for attaching router with external network gateway

In the CI environment, whenever testcases attaches a router with
external network gateway sometimes it throws the error "No more IP
addresses available on network". This could be due to stale floating
ip's exist in the deployment which is cleaned up now before executing
sdnvpn testsuite.

JIRA: SDNVPN-212

Change-Id: I027a833f13b1bdddd83dc8c7e7f7292e20b10bb8
Signed-off-by: Periyasamy Palanisamy <periyasamy.palanisamy@ericsson.com>
(cherry picked from commit 1bf65fd6d92cdf2c0ad694662cbc12b7764e3364)