Fix all occurrences of format-strings without any interpolation. pylint
warns about those (and for some reason did not do so for our modules).
A followup will fix the pylint tests, so make sure all the warnings are
resolved.
Make sure access to the shared ostree metadata is locked properly. This
is the default since 2018.5, but lets be explicit here. This also makes
sure that the option exists and the local version supports locked and
protected access.
It is unclear whether the `ostree init` honors that as well. It really
should, and if it doesn't we can always report it upstream.
Instead of verifying the gpg signature when pull from the actual
remote source into the local cache, verify the commit when it is
being pulled from the local cache into the output directory. This
ensures that the signatures are checked against the provided keys
even when the commit was already in the cache and at that time
the key might have been different.
NB: ostree expects the signature to be present on the remote at
the *target* repository, i.e. in our case the output repository.
The keys are therefore attached to a temporary remote that is
created at the output repository with the same name/id that is
used for the actual remote.
Add a new `gpgkeys` option that, if set, must contain a list of
public keys. These keys will then be used by ostree to verify
signed commits when pulling from the remote. If the `gpgkeys`
option is missing, no verification will be attempted.
This source can be used to fetch ostree commits. The commits are
accessed via their commit is. The only option currently is `url`,
given for each commit, that will be used as the location of the
remote. A cache repository, that will be created if necessary,
acts as an intermediary, so remotes will be added with `name` as
the identifier to it and commits are pulled into that. In the
output directory another repository will be created as 'repo' and
the requested commit pulled into that from the cache repository via
a local pull.
