cli: do not prompt when there is no tty available on stdin

[mvo5]

This is a bit of a drive-by. I noticed that inside travis
snapcraft 3 hangs with the following message:

Support for 'LXD' needs to be set up. Would you like to do that it now? [y/N]:

It seems like we should only prompt here if stdin is attached
to a tty and otherwise just fail early. This should fix hangs
in e.g. scripts.

[zyga]

Looks good to me :-)

[sergiusens]

Interesting, last I read click.confirm was supposed to take this into account. Mind if I take this further and centralize this change considering that we have https://github.com/snapcore/snapcraft/blob/master/snapcraft/internal/indicators.py#L107?

[mvo5]

@sergiusens When I played around with redirection it seems like click.confirm() does not take tty into account. But caveats apply, I did not look deeply and did not report an upstream bug (probably a good idea though). Go wild on this, this PR is just a drive-by as I was slightly annoyed by this hang in travis.

[sergiusens]

Thank you for the contribution, we will look into consolidating these scenarios into a follow-up PR

[codecov-io]

Codecov Report

Merging #2570 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2570      +/-   ##
==========================================
+ Coverage   89.05%   89.06%   +<.01%     
==========================================
  Files         205      205              
  Lines       13987    13988       +1     
  Branches     2118     2118              
==========================================
+ Hits        12456    12458       +2     
+ Misses       1081     1080       -1     
  Partials      450      450

Impacted Files	Coverage Δ
snapcraft/cli/lifecycle.py	86.95% <50%> (+0.09%)	⬆️
snapcraft/internal/elf.py	80.71% <0%> (-0.36%)	⬇️
snapcraft/internal/pluginhandler/_plugin_loader.py	85.29% <0%> (+1.96%)	⬆️

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