don't use activesupport methods like present?
skip agents with nil instead of attribute hash

To support previous defined agents (e.g. the default agent from cookbook
attributes), the hash can be replaced by nil. The recipe ignores these
agents.
correct group of teamcity installation
optimize agent source download behavior

* do not remove .zip per cookbook (cache-directory cleaning is task of chef)
* use md5 digest of server_url instead of agent name to avoid source misusages