Bugs 995550,996751,998704: Multiple related throttler issues

https://bugzilla.redhat.com/show_bug.cgi?id=995550
https://bugzilla.redhat.com/show_bug.cgi?id=996751
https://bugzilla.redhat.com/show_bug.cgi?id=998704

This PR fixes multiple issues with throttler
- [Bug995550][1] - Fixed the logic to not throttle already boosted gears
- [Bug996751][2] - Force throttler to only restore applications under a certain threshold (different from throttle threshold)
- [Bug998704][3] - NoMethodError when checking utilization for certain
  gears

[1]:  https://bugzilla.redhat.com/show_bug.cgi?id=995550
[2]:  https://bugzilla.redhat.com/show_bug.cgi?id=996751
[3]:  https://bugzilla.redhat.com/show_bug.cgi?id=998704