Add -x/--make-ha option to app-create

- Run the following command to make an scalable application HA

   rhc app-create your-app-name cartridge-name -s -x
        OR
   rhc app-create your-app-name cartidge-name -s --make-ha

- Check if you have an HA-proxy instance on each of the 2 gears created by running the following command-

   rhc app-show your app name --gears

Bug - 1139374
Bugzilla Link- https://bugzilla.redhat.com/show_bug.cgi?id=1139374

  Add testcases for -x/--make-ha

- Check if exception is raised  when there's any issue while making an application HA

- Check that -x/--make-ha is allowed only together with -s/--scalable.

- Check when make-ha is not supported by the server

- Mock MAKE_HA link conditionally