build: select_ubuntu_repo: break on err

build: select_ubuntu_repo: break on err

Stop ISO build process if no sane Ubuntu mirrors are available,
instead of continuing with an empty URL, which is bound to fail
later.

While at it, skip re-assesing the output of select_ubuntu_repo.sh if
the URL var already is set.

Also, stderr and stdout were both captured in UBUNTU_MIRROR_URL,
so disable debug by default, and only print debug info in case no
valid mirror is found.

v2 -> v3:
- allow "One hour behind" mirror state to be considered, since
  we already accept "Six hours behind" mirrors;

JIRA: FUEL-240

Change-Id: I791942da234304528ff5cae1891415602e1a62b2
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>