Fix randomly occurring plugin installation failures

What used to fail at

  expected #<TrueClass:2> => true
  got #<FalseClass:0> => false

  Compared using equal?, which compares object identity,
  but expected and actual are not the same object. Use
  `expect(actual).to eq(expected)` if you don't care about
  object identity in this example.

   (RSpec::Expectations::ExpectationNotMetError)
  ./features/step_definitions/plugin_steps.rb:6:in `install_plugin!'

was caused by timeout issues installing the plugin. Occasionally, it can take
~200 seconds for plugin to log the fact it was installed or even appear in
/pluginManager/installed instead of usual less than 10. Let's wait until it
happens.