feat(junos): improve _convert_to_set_commands

[Frazew]

This is a proposed follow-up for #125 to make the parsing of set-based configurations more robust:

• properly skip line comments
• use self.options["negation"] instead of the hardcoded delete when checking if the line is already a "set" line
• use the block indicators {/} instead of the indentation to build the configuration tree
• various minor implementation details
  • split("\n") -> splitlines(): to handle different line breaks
  • replace(";", "") -> .rstrip(";"): rstrip feels safer in that it doesn't remove a semicolon appearing elsewhere in the line (not sure it could happen to have a string containing a semicolon but better safe than sorry?)
  • move if self.options["syntax_style"] == "juniper": out of the function to remove unnecessary indentation and improve readability

I believe relying on indentation as is currently done could lead to unexpected results (if the generated configuration is not perfectly indented for example) especially since the configuration format provides a way to traverse the tree using braces and doesn't rely on indentation to be parsed. This new implementation can also properly error out on invalid configuration (unterminated blocks / extra braces).

I'm curious to know if there was a specific reason I'm not seeing to use indentation instead of parsing the blocks directly?

[jtdub]

LGTM! Thank you for your contribution!