Allow module to co-exist with other uses of its dependencies

[dspinellis]

If other modules also use the apache and mysql classes, duplicate declaration errors come up. Here is an example:

Duplicate declaration: Class[Apache] is already declared in file /etc/puppet/modules/web/manifests/init.pp at line 8; cannot redeclare at /etc/puppet/modules/mediawiki/manifests/init.pp:61 on node ...

[christabor]

I'm also seeing this. It is possible to namespace unique "one-off" instances of mysql, etc... as one solution. At the very least, it might be good to specify the requirements in the README (for example, memcached is a requirement.)