refactor the lvm driver, fixed some bugs

[xxwjj]

What this PR does / why we need it:

1. Removed some un-used codes in lvm driver.
2. Fixed two bugs: osdsdock bug parsing vgs size due to locale-dependent number format #608 Delete volume snapshot got an error #583
3. Sine the lvm.go file is more and more complex, I separate it into two files: lvm.go cli.go.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

[coveralls]

Coverage decreased (-0.5%) to 39.436% when pulling ed045cf on xxwjj:lvm_bugs into 3acb0c2 on opensds:development.

[leonwanghui]

LGTM