Allow non-interactive execution of install_server
Allow non-interactive execution of install_server

This PR adds the ability to execute the installation script non-interactively, useful for automated provisioning scripts such as Chef, Puppet, Ansible, Salt, etc.
Simply feed the environment variables into the install script to skip the prompts.

For debug and verification purposes, the script will still output the selected config variables.
The plus side is that the environment variables also support command substitution (see REDIS_EXECUTABLE).

```
sudo REDIS_PORT=1234 REDIS_CONFIG_FILE=/etc/redis/1234.conf REDIS_LOG_FILE=/var/log/redis_1234.log REDIS_DATA_DIR=/var/lib/redis/1234 REDIS_EXECUTABLE=`command -v redis-server` ./utils/install_server.sh

Welcome to the redis service installer
This script will help you easily set up a running redis server

Selected config:
Port           : 1234
Config file    : /etc/redis/1234.conf
Log file       : /var/log/redis_1234.log
Data dir       : /var/lib/redis/1234
Executable     : /usr/local/bin/redis-server
Cli Executable : /usr/local/bin/redis-cli
Copied /tmp/1234.conf => /etc/init.d/redis_1234
Installing service...
Successfully added to chkconfig!
Successfully added to runlevels 345!
Starting Redis server...
Installation successful!
```
JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis