Reduce polling interval in feature spec

What does this MR do?

Optimises spec/features/clusters/installing_applications_shared_examples.rb

Fixes flakiness in job (following http://johnpwood.net/2015/04/23/tips-and-tricks-for-dubugging-and-fixing-slowflaky-capybara-specs/)

Details

For $ time ss spec/features/groups/clusters/applications_spec.rb spec/features/projects/clusters/applications_spec.rb spec/features/admin/clusters/applications_spec.rb, Before:

Finished in 10 minutes 2 seconds (files took 2.67 seconds to load)

After:

Finished in 4 minutes 0.3 seconds (files took 4.81 seconds to load)