Add admin.getInstance

[pinglamb]

This PR is for kicking off #173.

An deployedProxyAdmin method is added to admin module to get ProxyAdmin instance (feel free to suggest names). My use case is simply to get the address programmatically without fs.readFileSync the manifest.

[pinglamb]

@frangio Please feel free to suggest the proper naming and code structure. Thanks!

[frangio]

Thanks @pinglamb! What is the reason for exposing it as a function (i.e. upgrades.admin.deployedProxyAdmin()) rather than just a getter (upgrades.admin.instance)?

[pinglamb]

It seems that if it is an instance, once hre.upgrades is called it will try to load the ProxyAdmin object. For example, if you are using it for the first time, it will throw the No ProxyAdmin ... error. Also I am not sure if there will be other issues if we are caching the contract object with hre.upgrades. I guess the safest is loading it on demand? What do you think?

[frangio]

Yes that makes sense. I think the function name getInstance sounds better to me. It's used as upgrades.admin.getInstance().

We have to add the same function in the Truffle plugin. The code is very similar. Can you do these changes?

[frangio]

Thank you @pinglamb! I've changed the tests a little to give us more confidence that the return value is correct.