Set default for MiningConfig.MinGasPrice to zero

[jmederosalvarado]

Changes:

• Changes the default value for MiningConfig.MinGasPrice to zero on all networks except for gnosis/xdai

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe): Config

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

[LukaszRozmej]

Now we will accept free transactions by default, do we want it or move to 1.Wei?