Enable configuring tcp keepalive

Added a function set_tcp_keepalive in network_utils.py to set configuration of
tcp keepalive parameters. Projects can call this function and set values in
them, if needed.

Also added unittests to test this function.

Implements blueprint configure-tcp-keepalive
Change-Id: I8b978a604f29953911c2fe4bad74e28e5ba74c35
Merge "Enable configuring tcp keepalive"