Vault client caching

[mparent61]

@collin this is my thought on vault caching, still needs tests + such

[mparent61]

@collin @MichaelButkovic @benghaziboy

Suggestion for Vault REST endpoint caching, what do you think?

Idea is to add 2 layers of caching:

• "short-term" caching to avoid overwhelming Vault service (~1 min)
• "backstop" caching (I made up the name) to fallback on if Vault endpoint is temporarily unreachable (~60 min)

Short-term seems like a no-brainer, the "backstop" concept could use some feedback.

[camsom]

niiiiiiice