Test S3 backupstore in a cluster sitting behind a HTTP proxy

^{Related issue: 3136}

Requirement:

Set up a stand alone Squid, HTTP web proxy
- To configure Squid proxy: a comment about squid config
- If setting up instance on AWS: a EC2 security group setting
S3 with existing backups

Steps:

Create credential for Backup Target

 $ secret_name="aws-secret-proxy"
 $ proxy_ip=123.123.123.123
 $ no_proxy_params="localhost,127.0.0.1,0.0.0.0,10.0.0.0/8,192.168.0.0/16"

 $ kubectl create secret generic $secret_name \
 --from-literal=AWS_ACCESS_KEY_ID=$AWS_ID \
 --from-literal=AWS_SECRET_ACCESS_KEY=$AWS_KEY \
 --from-literal=HTTP_PROXY=$proxy_ip:3128 \
 --from-literal=HTTPS_PROXY=$proxy_ip:3128 \
 --from-literal=NO_PROXY=$no_proxy_params \
 -n longhorn-system

Open Longhorn UI
Click on Setting
Scroll down to Backup Target Credential Secret
Fill in $secret_name assigned in step 1. and save setting
Go to Backup
Restore from existing backups and watch the volume become ready

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