[2.11.x][NEXUS-7596] Only activate Yum capabilities if create/mergerepo are available

https://issues.sonatype.org/browse/NEXUS-7596
http://bamboo.s/browse/NX-OSSF332
if this is used to build the command-line arguments to execute, this may break if any of these paths have spaces?  Granted not common on unix platforms, but still... a list of args is probably safer generally.