Revert "This changeset adds support of max_instances and available_instances.  Resource manager tracks all registered instances, except those in the DELETED state.  Its could_reserve? and numbers reserverable methods responds positively when the total number of undeleted instances fall under the max_instances limit."

This pull request was merged in prematurely, and since we still
have some concerns about this feature and its implementation, we've
decided to revert it for the time being. Please see additional details
on the GitHub PR discussion (https://github.com/cloudfoundry/dea_ng/pull/85).

This reverts commit c0202801f7599d47540fd62adc6f21fef4790bd4.

Signed-off-by: Michael Maximilien <maxim@us.ibm.com>