default.inv: Add note about problems with mixing hostnames/IP addresses

default.inv: Add note about problems with mixing hostnames/IP addresses

Added a note about risks with mixing IP addresses and hostnames
in inventory file.
If the same host will be called in inventory file using both its
hostname and IP address then problems may occur during step
where dependencies are installed (ansible thinks that there
are 2 independent hosts and will try install dependencies
for both of them in the same time - it may fail be because
of apt/yum synchronization locks).

Signed-off-by: Radoslaw Jablonski <radoslawx.jablonski@intel.com>
Change-Id: I2cd1ea293fadfb12f78c03028cafec993dadbe2d