Revisit the way how installer adds the administration user to the EAP

Since it doesn't seem that WFCORE-306 will make it into EAP 7.0, we should revisit the way how installer adds the administration user to the EAP.

Looking to default mgmt-users.properties file after unzipping the EAP, there is commented out text which could help users with the usage of add-user utility, see http://pastebin.test.redhat.com/325698.

Adding a user via add-user script will append the user to the end of the file, also notice the REALM_NAME which is used by add-user utility, see http://pastebin.test.redhat.com/325699

Now run the installer, create a "admin" user during installation and see the differences in mgmt-users.properties files between unzipped and installed distributions, see http://pastebin.test.redhat.com/325701 for mgmt-users.properties file after installation procedure.

things of interest
- all commented out text is missing
- REALM_NAME is missing

Looking to the add-user script, user is created by following command:

java -jar ${JBOSS_HOME}/jboss-modules.jar  -mp ${JBOSS_HOME}/modules org.jboss.as.domain-add-user $USER_NAME 

Since the EAP is already installed by the time of user creation, installer could utilize domain-add-user utility as well making the user creation consistent with zip distribution.