For parameters that are required as part of an API, prefix them
by "_" to make it clear they are unused

refactor: Use [a, b, *c] instead of [a, b] + c when building list

See merge request BuildStream/buildstream!1200

This pattern is available from python3.5 and provides a simpler
understanding of what is going on

runcli: rm unused 'color' and '**extra' params

See merge request BuildStream/buildstream!1198

The only customer for 'invoke' is the 'run' method on the same class.
Make it private and simplify it's interface.

Avoid disappointment by removing unused 'color' and 'extra' parameters
from 'invoke'.

These appear to have been here to mirror the interface of
click.testing.CliRunner.invoke(), which this class replaced.

cascache: limit 'infinite' cache to volume size

Closes #906

See merge request BuildStream/buildstream!1196

Make sure we don't exceed the size of the volume when calculating an
'unlimited' quota.

Note that at present, cache_size is an estimate that will be the same
as the cache_size or larger. In my own case, I have a volume dedicated
to the cache, so it was easy for the cache_quota to exceed this. This
prevented me from running BuildStream, although I had plenty of free
space.

buildelement: rm unused 'cmd_name' argument

See merge request BuildStream/buildstream!1197