NEXUS-5956: Yum Proxy capability

That simply removes stale YUM MD when repomd.xml changes
NEXUS-5965: Preparations for IT
NEXUS-5956: Client parts for new capability
NEXUS-5956: IT
NEXUS-5956: Simplify condition when to kick in
NEXUS-5956: Make support class return exact types, to ease life
NEXUS-5956: IT was wrong, do not delete but make proxy always go remote
NEXUS-5956: Tidy, as requests and yum constants are not quite aligned
Merge remote-tracking branch 'origin/master' into NEXUS-5956-yum-proxy

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>

Conflicts:
	plugins/yum/nexus-yum-repository-plugin/src/main/java/org/sonatype/nexus/yum/internal/YumRegistryImpl.java
[NEXUS-5956] Introduce YumProxy (similar to YumHosted/YumGroup introduced in NEXUS-5957)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5956] Yum proxy metadata uses "item max age" not "artifact max age"

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5956] Do now fail if we have an ItemNotFoundException as it may be that "/repodata" is not yet present

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5956] Ignore walking exceptions and just log a warning

The fact that we could not clean up some files should not stop us from getting yum metadata from remote

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5956] Organize imports

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #200 from sonatype/NEXUS-5956-yum-proxy

NEXUS-5956: Yum Proxy capability