Added the flag to set the external gateway in nova.
The flag is set default config page.
Taking the value from default config page the external gateway will be updated
in nova.

Change-Id: I0d164157960761eeebf4a1aaf066eda156bfc705
Merge "Added the flag to set the external gateway in nova. The flag is set default config page. Taking the value from default config page the external gateway will be updated in nova."