Found a way to achieve what I wanted.
Apple has introduced an interesting tool with Xcode 6!
simctl
simclt allows you to control the simulator that are running.
run xcrun simctl to get the list of available subcommands. Lots of new options to play around with.
Now to do what I wanted, here is the command to launch the simulator:
xcrun instruments -w "iPhone 5 (8.0 Simulator)"
-w is to provide a device type and to get the list of available devices.
Just execute this:
xcrun instruments -s
After launching the simulator you can control it using simctl
To install your app:
xcrun simctl install booted <app path>
To launch the app:
xcrun simctl launch booted <app identifier>
Hope this helps.