(java-processor) Use the same logic as the real confd to retreive values

[looztra]

The current behaviour of the confd-maven-plugin is to:

• start from keys
• upper case everything
• replace / by _
• use this in templates

The behaviour of the confd binary is the other way:

• upper case the "prefix" keys
• search for env vars that start with these prefixes
• replace _ by / and lower case everything
• use this is templates

Hence, a key with name /my/ns/httpPort will work in the confd-maven-plugin but not with pure confd (because MY_NS_HTTPPORT is converted in /my/ns/httpport)

The confd maven plugin should use the same behaviour as the confd binary to allow users to fail fast