Add support for reformatting YAML files

[cognifloyd]

I want to introduce a feature to ansible-lint: It should be able to fix issues identified by rules. The feature in this PR is a pre-requisite to introducing that.

In order to fix any issues identified by rules, we will have to read each YAML file with ruamel.yaml and then write it out. Reading/writing YAML files with ruamel.yaml standardizes spacing and quotes while preserving comments. In other words, it does very basic YAML reformatting which might fix some of the issues identified by yamllint. Fixing most of the issues identified by ansible-lint (including at least some of the yamllint-identified issues) requires transforming the files in some way. This PR only implements the read+write part, leaving the actual rule transforms for future PRs.

To use this (opt-in) mode, run ansible-lint --write (UX discussion summarized in #1910). That makes ansible-lint pass all YAML files it checks through our customized ruamel.yaml.YAML instance.

ansible-lint --write reports a count of the number of modified files, which looks approximately like this:

Modified 42 files.
Finished with 0 failure(s), 0 warning(s), on 100 files.

Notes:

• This is and must be opt-in. ansible-lint will not modify any files unless you ask it to.
• The tests here revealed a YAML formatting issue not accounted for in Configure/Customize ruamel.yaml Loading and Dumping #1927 that moved full line comments instead of just eol comments. 808e050 fixes that issue.

Other merged PRs prepared the basis for this one (which replaces #1828):

• Enable Lintables to be modified #1884
• Add --write option for upcoming reformat/transform features #1926
• Add formatting fixtures #1934
• Add tool to generate formatting fixtures #1936
• Configure/Customize ruamel.yaml Loading and Dumping #1927
• Copy files for Transformer tests without any changes #1944
• Manage YAML comment indentation #1946
• Configure FormattedYAML using YamllintConfig #1947
• Add YAML explicit start --- throughout repo #1948
• Standardize yaml quotes #1950
• Standardize yaml booleans #1951
• Misc YAML formatting fixes #1952

[ssbarnea]

Please rebase :P

[cognifloyd]

1. If yaml is in the skip_list, then should --write throw an error? No other transforms will be able to have any affect without reformatting the yaml file. In any case, I'm looking at how to account for the skip list.
2. Oh. I thought I had a safety on there for that issue. Looks like I missed handling that in this iteration. If the file is not a map or a list, then it should not try to write it because ruamel.yaml cannot preserve comments when the file loads as "None" (which an empty vars file will do). I'll push the fix in a bit.
3. The empty flow issue { } vs {} is addressed in Manage YAML comment indentation #1946.

[cognifloyd]

1. If --write and yaml is in skip_list, then the Transformer no longer runs. Instead a message gets printed to explain why nothing was modified.
2. I added the empty file safety check.