Use standard locale in list_partitions

parted function can output values which cannot be parsed by
_PARTED_PRINT_RE regexp in disk_partitioner module when using for
example ukrainian locale, as comma is used to separate integer and
fractional parts of float numbers. Standard 'C' locale should be used
to run parted.

Change-Id: Ic5b043796d548d91fb57a7ee34fe2e5a3a09088c
Closes-bug: #1371079
Merge "Use standard locale in list_partitions"