Update ironic-lib min required version

The addition of the qemu-img module  that is used in ipa was
introduced in ironic-lib 6.0.0

Change-Id: I65b283d9abcfe2aa961baf9f686391c8dcc71d6b

Author: elfosardo

requirements.txt

@@ -15,7 +15,7 @@ pyudev>=0.18 # LGPLv2.1+
 requests>=2.14.2 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 tenacity>=6.2.0 # Apache-2.0
-ironic-lib>=5.1.0 # Apache-2.0
+ironic-lib>=6.0.0 # Apache-2.0
 Werkzeug>=2.0.0 # BSD License
 cryptography>=2.3 # BSD/Apache-2.0
 tooz>=2.7.2 # Apache-2.0