Run `zebrad start` by default

[mpguerra]

Is your feature request related to a problem? Please describe.

During our first alpha user studies (#1501) we discovered that users were confused about the exact command that they should execute in order to run zebrad and in light of their knowledge of how zcashd and bitcoind are run.

Describe the solution you'd like

In order to improve usability, we should:

• [ ] run the start command by default when a user runs zebrad

Describe alternatives you've considered

We are already reviewing instructions in the README In #1617

[oxarbitrage]

I am not sure if this is possible with abscissa : https://github.com/iqlusioninc/abscissa/issues/101