Integration tests for "elemental3 customize"

[ipetrov117]

This PR suggests an implementation for integration tests that relate to the elemental3 customize command.

About the integration tests themselves:

1. What is the use-case that is being tested? - as discussed, the tests are handling the linux-only image use-case with the following customizations enabled:
   • Statically defined network.
   • Custom systemd service.
   • Custom kernel cmdline.
   • FIPS enabled.
2. What are the actual tests? - tests are fairly simple and cover the following:
   • Are we running the expected OS?
   • Are we running the expected network?
     • Has the hostname been defined correctly as part of the static network definition?
     • Check various network properties (e.g. is it up, is it of the expected type, does it hold the expected IP, etc..)
   • Are we running the OS with FIPS enabled?
   • Has the custom kernel cmdline been propagated?
   • Has the custom systemd service been correctly setup?
3. How do the tests affect the current workflow? - test are run after the existing elemental3ctl install tests. Between each test we do a cleanup to ensure that the run_vm.sh script can successfully build the next VM. The alternative to this was doing a cleanup after all tests, but this proved to be less effective and readable, as multiple needless properties and configurations had to be made so that run_vm.sh could boot 3 VMs next to one another.

What is left? - Currently these tests are running with a release manifest that is taken directly from devel:UnifiedCore:Tumbleweed. This needs to change, so that the tests take a release manifest from the respective OBS project that was created for the respective PR. To do this we would need to first land #319. After that the only change to this PR would be a reference to the respective release manifest, which should be a minimal change. Nevertheless, until that happens this PR will stay in Draft.