Dialog seeding changes necessary for the service dialog yaml import.

Support service dialogs imported from a linked directory.
Changed dialog seeding to accommodate sub-directories and added tests.

https://bugzilla.redhat.com/show_bug.cgi?id=1163875
These ```context``` and ```it``` blocks don't seem to make sense like this. If there is different criteria for "default directory", "sub-directory", "symlinked", then those should be separate contexts.
Ex:
```ruby
context "when the service dialog import yaml file is in the default directory" do
  # Some sort of setup as to how it's in the default directory

  it "delegates to the dialog import service with the correct file name(s)" do
    # Test(s) here
  end
end

context "when the service dialog import yaml file is in a sub directory" do
  # Other setup

  it "delegates to the dialog import service with the correct file name(s)" do
    # Test(s) here
  end
end
```

If there is no differing setups (which I think how it is because we're using fixture files), then we could have 3 separate tests structured with no context like so:
```ruby
it "delegates to the dialog import service with a file in the default directory" do
  # Tests
end

it "delegates to the dialog import service with a file in a sub directory" do
  # Tests
end

it "delegates to the dialog import service with a symlinked file" do
  # Tests
end
```

Also I would change ```yaml_string``` to be what you're passing to ```ALL_YAML_FILES```, ala:
```ruby
let(:all_yaml_files) { test_file_path.join("{,*/**/}*.{yaml,yml}") }
```