--skip-drop-table by default

[chumaltd]

Hi, I'd like to have --skip-drop-table option by default. For dropping, --drop-table.
#105 proposed about this, but looks not discussed.

For Data safety

Obviously, resulting tail risk differs between default options.

• Missing --skip-drop-table causes permanent DATA LOST.
• Missing --drop-table requires just rerun.

For Table Partitioning

I tested Table Partitioning on PostgreSQL.
With --skip-drop-table option, ridgepole works mostly well.

Table Partitioning has multiple tables called "partitions" that actual data resides.
Partitions schema derives from a single abstruct "partitioned table", so ridgepole doesn't have chance to migrate "partitions".

With --skip-drop-table, we can migrate "partitioned table" as just plain tables and then followed by its partitions.
(I don't mean that ridgepole can migrate a plain table into a partitioned table. It requires manual operations anyway.)
With partitioning, we have many arbitrary partitions, which faces DATA LOST risk with current ridgepole default behavior.

Table partitioning is just a case, and we will have situations that ridgepole should let several tables go.

[coveralls]

Pull Request Test Coverage Report for Build 1200239096

• 0 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-19.3%) to 77.983%

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Totals
Change from base Build 1176533959:	-19.3%
Covered Lines:	928
Relevant Lines:	1190

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1200239096

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.236%

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Totals
Change from base Build 1176533959:	0.0%
Covered Lines:	1161
Relevant Lines:	1194

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💛 - Coveralls

[winebarrel]

Thank you!