maven settings script improvements

The following enhancements would improve our maven support in settings.xml via maven.sh and maven-settings.sh (was maven-repos.sh).

script fixes:

• unset missing single-repo-scenario env vars
• remove unused do_rewrite local script variable
• fix associated bats tests broken from changes in CLOUD-2477

repositories ( https://maven.apache.org/ref/3.5.4/maven-model/maven.html#class_repository ):

• support name element in repository (default = repository id)
• support checksumPolicy element in releases and snapshots (default = warn)

servers ( https://maven.apache.org/settings.html#Servers ):

• support filePermissions in server element
• support directoryPermissions in server element

functionality:

• add configuration of mirrors and proxies to non-s2i builds (by merging maven.sh and maven-repos.sh)