Add variable substitution to fields within a Projected Volume

[jlpettersson]

Expected Behavior

That I can use projected volumes, e.g. for more secure ServiceAccountTokens (that expire and may have single-audience), or using a Secret or ConfigMap mounted to a single directory.

I also want to name my Secret, ConfigMap and ServiceAccountToken how I want in my environment.

Proposal

Add variable substitution for the most common fields within a Project Volume.

Use cases:

• git credentials from a projected volume
• service account token - rotated - from a projected volume
• secret from a projected volume

/kind feature
/assign