connman: add network interface management code in service file

JIRA: SB-3096

This was installed as separate service file previously,
make the changes in source file directly.

Signed-off-by: Srikanth Krishnakar <Srikanth_Krishnakar@mentor.com>
Merge pull request #255 from skrishnakar/install-connman-service

connman: add network interface management code in service file