Add a AbstractMetadataRepository to remove duplicated code
[MRM-1390] Allow searching JCR store for generic metadata with getArtifactsByMetadata
[MRM-1390] Add getArtifactsByMetadata REST method
[MRM-1390] Make repositoryId optional in getArtifactsByMetadata
[MRM-1390] Ignore testGetArtifactsByMetadataSingleResult until implemented in file and cassandra backends