Very basic Oracle support for Zabbix server.

Hello,

### Intro 

We have a need to connect to an Oracle database since most of our DBAs know Oracle - it is the most supported RDBMS for us to use. I have modified the cookbook to support connecting as an *oracle client* only. These changes make no attempt whatsoever to configure the Oracle *server* (yet - maybe in a future PR). 

I accept this may not be merge worthy as it currently stands but I thought I'd pass it along to the community in case someone else finds it useful.

This is all not heavily tested outside of our environment so feedback welcome.

### Prerequisites

As alluded to above - this requires the following

* Fully configured Oracle server with the Zabbix tablespaces, schema and user ready to go
* Oracle instantclient RPMs available to download - set the appropriate attributes as described in the [oracle-instantclient](https://github.com/juliandunn/oracle-instantclient) cookbook
* Oracle instantclient attributes set allowing successful installation of the instantclient binaries

### Attributes

* node['zabbix']['database']['install_method'] - set this to "oracle"
* node['zabbix']['database']['dbname'] - set this to the Oracle SID
* The rest of the attributes serve the same functions