fix proxy not being used to test connectivity

[anjalshireesh]

Description

The code to check SUBNET reachability was using default client without proxy, even when the proxy is set.

How to test this PR?

• Register a cluster with subnet
• Configure access to subnet only via proxy
• Set the proxy using mc support proxy set {alias} {proxy-url}
• Check that subnet related commands like mc support diag work fine in online mode (without --airgap)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here