Optimize Vagrant setup

- define grains to match optimized gvasalt repository
- define minion configuration in separate file
- drop custome salt/bootstrap.sh
- adapt Vagrantfile to use the custom grains file
- increase memory size of created VM to allow the salt provisioning to
  succeed

salt/grains

@@ -0,0 +1,14 @@
+gnuviechadmin:
+  user: vagrant
+  group: vagrant
+  amqpuser: ldap
+  appname: gvaldap
+  checkout: /vagrant
+  fullname: LDAP
+  giturl: gituser@nextgit.gnuviech-server.de:gnuviech/gvaldap.git
+  home: /home/vagrant
+  update_git: False
+roles:
+  - vagrant
+  - ldapserver
+  - gnuviechadmin.gvaldap