Allow a role other than admin* be used as the admin role

We've got some strange requirements that dictate using a "super admin" role with adminrole and have been trying to activate it as part of an install profile. The way the hook_install() code is written it's difficult to avoid having it create the "administrator" role and grant it all the permissions. The attached patch is an attempt to avoid this. Now our install profile can set the variable before the module is installed and use the correct role.