installation: CentOS7 instruction fixes

• BETTER Improves NodeJS installation instructions for CentOS7 by switching to using upstream NodeJS procedures instead of EPEL. This permits to update NPM just like for Ubuntu.

• BETTER Adds more SELinux-oriented installation commands that are necessary for CentOS7 boxes that use SELinux.

• FIX Fixes firewall installation instructions for CentOS7.

• FIX Amends upgrading of virtualenvwrapper fixing Ubuntu14 installation issue.

• Switches to centos/7 Vagrant base image.

• Removes provisioning of uuid package on CentOS7 that is no longer necessary.

• Alters order of "create instance", "populate instance" and "start instance" commands for Vagrant based installation technique.

• Note that the three installation techniques (Docker, Ubuntu14 via Vagrant, CentOS7 via Vagrant) are all successfully passing at the time of this commit.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>