ostree plugin should raise an error if ostree < v2017.8 is installed
## Summary
Builds fail with a strange error if the ostree version is not new enough
## Steps to reproduce
Try to build anything with an older ostree (even if it doesn't use the plugin at all)
## What is the current bug behavior?
```
~/gnome-build-meta$ bst build --track-all sdk/gtk+-3.bst
[--:--:--][][] START Build
[--:--:--][][] START Loading elements
[00:00:03][][] SUCCESS Loading elements
[--:--:--][][] START Resolving elements
[00:00:01][][] SUCCESS Resolving elements
[--:--:--][][] START Initializing remote caches
[00:00:00][][] SUCCESS Initializing remote caches
[--:--:--][][] START Resolving cached state
[--:--:--][][] BUG 'Repo' object has no attribute 'reload_config'
Traceback (most recent call last):
File "/home/niels/.local/bin/bst", line 11, in <module>
sys.exit(cli())
File "/home/niels/.local/lib/python3.5/site-packages/click/core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/_frontend/cli.py", line 162, in override_main
standalone_mode=standalone_mode, **extra)
File "/home/niels/.local/lib/python3.5/site-packages/click/core.py", line 717, in main
rv = self.invoke(ctx)
File "/home/niels/.local/lib/python3.5/site-packages/click/core.py", line 1137, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/home/niels/.local/lib/python3.5/site-packages/click/core.py", line 956, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/home/niels/.local/lib/python3.5/site-packages/click/core.py", line 555, in invoke
return callback(*args, **kwargs)
File "/home/niels/.local/lib/python3.5/site-packages/click/decorators.py", line 27, in new_func
return f(get_current_context().obj, *args, **kwargs)
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/_frontend/cli.py", line 330, in build
build_all=all_)
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/_stream.py", line 184, in build
dynamic_plan=True)
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/_stream.py", line 931, in _load
self._pipeline.resolve_elements(elements)
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/_pipeline.py", line 139, in resolve_elements
element._update_state()
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/element.py", line 1041, in _update_state
self.__update_source_state()
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/element.py", line 1909, in __update_source_state
source._update_state()
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/source.py", line 577, in _update_state
self.__consistency = self.get_consistency()
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/plugins/sources/ostree.py", line 169, in get_consistency
self.ensure()
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/plugins/sources/ostree.py", line 181, in ensure
self.repo = _ostree.ensure(self.mirror, True)
File "/home/niels/.local/lib/python3.5/site-packages/buildstream/_ostree.py", line 66, in ensure
repo.reload_config()
AttributeError: 'Repo' object has no attribute 'reload_config'
```
## What is the expected correct behavior?
```
$ ~/gnome-build-meta$ bst build --track-all sdk/gtk+-3.bst
Minimum required version for ostree plugin is v2017.8, please upgrade
```
## Other relevant information
- BuildStream version affected: `1.2.3`
issue
GitLab AI Context
Project: BuildStream/buildstream-plugins-community
Instance: https://gitlab.com
Before proposing or making any changes, READ each of these files and FOLLOW their guidance:
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Repository: https://gitlab.com/BuildStream/buildstream-plugins-community
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