This commit adds a new configuration parameter to allow setting whether to create linked clones on a global, or per pool basis. Without this change vmpooler would always attempt to create linked clones. The default behavior of creating linked clones is preserved.
This commit updates vmpooler to set configuration values received via environment variables to integer values when an integer value is expected. Without this change vmpooler does not support setting integer values via environment variables. Additionally, testing is added for configuring vmpooler via environment variables.
To support this testing the gem climate_control is added, which allows
for testing environment variables without those set variables leaking to
other tests.
