test: add optional --preserve-instance arg to integraiton tests
By default, integration tests destroy the test instances after each
test run. To aid debug and development of integration tests, support a
--preserve-instance argument which will leave the modified test instance
in a stopped state for further debug.
This adds 'combine_capture' argument as was present in curtin's
subp. It is useful to get interleaved output of a command. I noticed
a need for it when looking at user_data_rhevm in DataSourceAltCloud.
That will run a subcommand, logging its stdout but swallowing its stderr.
Another thing to change to use this would be in udevadm_settle which
currently just returns the subp() call.
Also, add the docstring copied from curtin's subp.
The pylxd project has a setup.py which defines install dependencies.
Those sub-dependendencies include pbr and requests which in turn have
package version conflicts. Since tox doesn't order dependencies installed,
serially install pinned urllib3 at 1.22 which supports both pbr deps and
requests deps of pylxd.
util: add get_linux_distro function to replace platform.dist
Allow the user to set the distribution with --distro argument to setup.py.
Fall back is to read /etc/os-release. Final backup is to use
platform.dist() Python function. The platform.dist() function is
deprecated and will be removed in Python 3.7
pyflakes: fix unused variable references identified by pyflakes 2.0.0.
A newer version of pyflakes (2.0.0) was released.
It identifed some unused variables that version 1.6.0 did not identify.
The change here merely fixes those unused variables.
