[NEXUS-5607] Mark Min to keep / Retention days fields as mandatory in Snapshot removal task

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #825 from sonatype/nexus-5607-snap-remove-mandatory

TRIVIAL: [NEXUS-5607] Mark Min to keep / Retention days fields as mandatory in Snapshot removal task