Add Host flag support

Since Memcached is able to listen on a specific port, we should
be able to choose which host to run the sensu check.
This patch add a new flag in the common fashion :
-h or --host with an IP or hostname as parameter
Add OpenStack Ceilometer checks

- Create OpenStack directory
- Create Ceilometer subdirectory in OenStack
- Copy checks script from Nagios

OpenStack is an Open-Source IaaS managment system.
Ceilometer is the metering service in OpenStack.
Add a new check for Keystone API

- Check if we can get a token
- Verify it's not too long
Rename "Nagios" to "Sensu" to avoid confusion
Merge pull request #250 from enovance/master

Add Host flag support