Fix conf file open mode when "tempest init"
Fix conf file open mode when "tempest init"

This commit fixes the conf file open mode when "tempest init" command
runs. The original open mode "w" truncates its file. We should use 'a'
for updating a file instead.

Change-Id: I033454950cf58615e212d8c9fdf96765cf45c454

openstack/tempest,masayukig/tempest,sebrandon1/tempest,Tesora/tesora-tempest,vedujoshi/tempest,bigswitch/tempest,cisco-openstack/tempest,openstack/tempest,Tesora/tesora-tempest,bigswitch/tempest,vedujoshi/tempest,Juniper/tempest,cisco-openstack/tempest,masayukig/tempest,sebrandon1/tempest,Juniper/tempest