prevent backup.mongodump.compression 'none' to be overwritten

[islue]

• in case mongodump can_gzip(), the non-default value 'none' will be overwritten. Rolled it back to the former logic. Figure me out if it is done on purpose.
• pip of recent version doesn't recognize --download-cache.

[timvaillancourt]

Thanks @islue.

The tool defaults to using 'gzip' with mongodump if the binary is able to provide that compression so this would be a breaking change to the code.

What I think needs to be done is add a 3rd compression type called 'auto', so we have these options:

1. 'auto' - automatically use compression if it's available (new default)
2. 'none' - disable compression
3. 'gzip' - use gzip encryption explicitly

I am able to implement this if needed.

The --cache-dir fix is already implemented in #129, to be rolled out with 1.0.2.

[timvaillancourt]

Hi again @islue.

I ended up implementing this 'auto' compression type in this branch as I ran into other problems that needed this fixed as well.

You can try this out on this branch: https://github.com/Percona-Lab/mongodb_consistent_backup/tree/1.0.2 which also includes the --cache-dir fix for pip.

The default compression is 'auto' and now 'none' should work. Please let me know if this fixes your concerns.

When in 'auto' mode this is logged and gzip is enabled if available:

[2017-05-11 16:43:15,630] [INFO] [MainProcess] [Mongodump:choose_compression:48] Mongodump binary supports gzip compression, auto-enabling gzip compression
[2017-05-11 16:43:15,632] [INFO] [MainProcess] [Task:compression:38] Setting Mongodump compression method: gzip

If compression is 'none' nothing is logged and compression is not used.

[timvaillancourt]

Closed by accident. Reopened

[islue]

@timvaillancourt It's OK for me. Thank you.