Tweak "quick install" instructions

1. Use curl for both master and minion, for consistency
2. Clean up introductory lead text
3. Demonstrate -A option with minion
4. Remove some redundant text.
5. Make capitalisation consistent.
6. Use “set up a Salt Master/Minion” rather than “install the Salt
Master/Minion” for usability. (“The Salt Minion” sounds weird when you
will have many of them.)
7. Embolden 'Salt Minion' and 'Salt Master' for quick scanning.
Merge pull request #10691 from stevage/develop

Tweak "quick install" instructions