nova.conf configurable gzip compression level

Currently only used by XenAPI.

Sets GZIP environment variable before spawning tar -cz to force the
compression level. Defaults to none, which means the GZIP environment
variable is not set and the default (usually -6) is used.

Implements: blueprint image-compression-mode

Change-Id: I02e136b7215a4f2cefc259f40bde9c1b205737ed
Merge "nova.conf configurable gzip compression level"