yaml.load missing required Loader on lunar

[Adam Collard]

 Building images for v3 streams
Traceback (most recent call last):
  File "/build/config/maas/bin/meph2-build", line 25, in <module>
    call_entry_point("meph2.commands.build_image.main")
  File "/build/config/maas/bin/meph2-build", line 22, in call_entry_point
    sys.exit(getattr(sys.modules[istr], ent)())
  File "/build/config/maas/meph2/commands/build_image.py", line 102, in main
    cfgdata = yaml.load(fp)
TypeError: load() missing 1 required positional argument: 'Loader'

As per CHANGES in pyyaml

* https://github.com/yaml/pyyaml/pull/561 -- always require `Loader` arg to `yaml.load()`

yaml.load() now requires a Loader argument

[Alberto Donato (ack)]

we should probably change to use yaml.safe_load() everywhere

[John Chittum (jchittum)]

took a quick look, because this is used all the way back to Bionic chroots. `yaml.safeload()` is supported in 3.12, so this approach should be safe across all the releases. I don't know the release history well enough to know when the interface may have changed. since Lunar is on 6.0 right now, seemed like a good idea to check.