Retry getting the list of service plugins

On systems that start both neutron-server and neutron-l3-agent together,
there is a chance that the first call to neutron will timeout. Retry upto
4 more times to avoid the l3 agent exiting on startup.

This should make the l3 agent a little more robust on startup but still
not ideal, ideally it wouldn't exit and retry periodically.

Change-Id: I2171a164f3f77bccd89895d73c1c8d67f7190488
Closes-Bug: #1353953
Closes-Bug: #1368152
Closes-Bug: #1368795
Merge "Retry getting the list of service plugins"