account for unique user group

[lattisimo]

Use id -gn to get the user group for curl|bash stackstorm installer.

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The issue I had was installing on a box that had LDAP configured and my user group is 'domain users' since the script just assumed the group matched the user so the chown command fails and the script stops executing. I can avoid the issue by 'sudo su' but I realized that if I get the user's group with 'id -gn' then there is no assumption of what that value should be. This would likely be relevant on the other scripts.