Replace environmant variables settings with global context

[igaw]

There are a handful of environment variables which control the library. With the introduction of the global context it should be possible to use it to configure the library.

library behavior:

• - LIBNVME_FORCE_4K
• - LIBNVME_MI_PROBE_ENABLED

testing:

• - LIBNVME_SYSFS_PATH
• - LIBNVME_HOSTNQN
• - LIBNVME_HOSTID