Reset tempdir config option after NestedTempfile fixture applied
Reset tempdir config option after NestedTempfile fixture applied

When running unit tests, tempdir config option is set to
tempfile.gettempdir() by default in ironic/common/utils, which
happens before fixtures.NestedTempfile() is applied, which causes it
to be a system tempdir. Reset its value after fixture is applied so
that nested directory would be used as 'root' directory for temporary
files.

Change-Id: I1e717c6e7b421ff212f3ef3d30577d24362e1c44
Closes-bug: #1489796

dims/ironic,bacaldwell/ironic,NaohiroTamura/ironic,openstack/ironic,openstack/ironic,SauloAislan/ironic,naterh/ironic,redhat-openstack/ironic,devananda/ironic,dims/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,ionutbalutoiu/ironic,pshchelo/ironic,hpproliant/ironic,pshchelo/ironic,SauloAislan/ironic,NaohiroTamura/ironic