Refactoring of diskimage-create script

The script is divided into blocks corresponding version of installed plugin.

Added options for specifying image type and hadoop version. By using option
you can choose image that you want to test instead of creating them all.

Change-Id: I5be5413e978acae26454a1a98825649f7d6498cd
Add CentOS 6.5 support

Edit doc for building CentOS cloud image

Change-Id: I1e77589a6faf11f898227de694323cead8e4ec3f