Fix bgpcep-1node-userfeatures tests

Python 2 & the network script (/etc/init.d/network) are not available on
CentOS 8. Update the Robot test script to address the incompabilities.

Update test scripts still using RESTCONF draft to switch to RFC8040
URLs, status code & data format.

Also, fix some URLs which were not properly defined in the Variables
section because it included an imported varialbe 'REST_API'.

Change-Id: Id52566f332455be7525166a462b47b5cf4c2d295
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>