WebUI-unit tests: npm is installed using dnf instead of included in image

[pvomacka]

npm package should be included in image from the beginning instead of installing it using dnf. The reason is that it might happen that mirrors with Fedora packages are not available and test fails, which might block from pushing pullrequest. Another advantage might be that we save some time, but compared to whole time needed for running Travis jobs it is insignificant.