Use safe_dump when saving charm metadata yaml

Ran into an issue using amulet with Python 2 where a unicode
string crept into the sentry metadata which resulted in a
tagged value that broke on juju-deployer loading the file.
Use correct variable name in get_relations
Merge pull request #4 from bz2/yaml_safe_dump

Use safe_dump when saving charm metadata yaml
Merge pull request #5 from bz2/trivial_get_relation

Use correct variable name in get_relations