[NXCM-4485] Set p2 temporary directory under "nexus-p2-bridge-plugin/tmp"

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #5 from sonatype/nxcm-4485-use-nexus-temp-dir

TRIVIAL: [NXCM-4485] Set p2 temporary directory under "nexus-p2-bridge-plugin/tmp"