Minor update to args parsing for installplugin.groovy

Minor change from System.properties to parsing args using CliBuilder. Mi...ght make the contract of the script clearer to users.
Provides a bit more usage information when running:
```
groovy ../../../nexus-oss/buildsupport/scripts/installplugin.groovy                                                                                                                                               
error: Missing required options: p, i
usage: groovy installplugin.groovy -i <nexus-install-directory> -p
              <nexus-plugin-bundle-zip>
A utility to install a plugin for development purposes
 -i,--install <arg>   Path to <nexus-install-directory>
 -p,--plugin <arg>    Path to <nexus-plugin-bundle-zip>
```