Support a complete ALTER TABLE statement in --alter

[shlomi-noach]

This PR supports a complete ALTER TABLE statement in --alter command line argument.

Until this PR, if one wanted to run a ALTER TABLE my_schema.my_table DROP COLUMN b, one would have to run gh-ost with:

• --database=my_schema
• --table=my_table
• --alter="DROP COLUMN b"

Starting this PR, gh-ost can deduce schema and table names from the --alter statement. It's now valid to:

• --ALTER="ALTER TABLE my_schema.my_table DROP COLUMN b"

as well as:

• --database=my_schema --ALTER="ALTER TABLE my_table DROP COLUMN b"

[shlomi-noach]

I did intend to first open this PR against https://github.com/openark/gh-ost ; but hit the wrong button... Since we're already here, I'm also happy to submit it upstream.