New proxy attribute to avoid single point of egress

The latest al-agent package supports --proxy(Linux) PROXY_URL(windows) to avoid a single point of egress. This value is optional for al-agent.

The al-agent package can be provisioned in the traditional way:

Linux:
/etc/init.d/al-agent configure --host "https://vaporator.alertlogic.com:443"

The agent package may also be provisioned using a --proxy parameter:
/etc/init.d/al-agent configure --proxy "schema://address:port" --host "https://vaporator.alertlogic.com:443"

Windows:
msiexec.exe /i 'agent package'"

The agent package may also be provisioned using a USE_PROXY parameter:
msiexec.exe /i 'agent package' USE_PROXY="schema://address:port"