and make conda_context argument mandatory in functions/methods.
This should help making sure the correct context (e.g. channels)
is used for all Conda operations.

Also remove unused is_target_available() function.

Conflicts:
	lib/galaxy/tools/deps/conda_util.py

The way we were modifying condarc in the ensure_channels_configured()
method was broken because we were appending the missing channels at the
end, so the final order may not be the one specified in
conda_ensure_channels .

Also:
- not modifying the user condarc is a much better approach
- overriding the channel list on the command line will make debugging
  easier