Fixing Neutron Subnet Details help text

Removing the '&lt;' and '&gt;' tags which were wrongly displayed in
the help text of the Subnet Details dialog for Neutron Networks /
Subnet create.

Closes-Bug: #1311820
Change-Id: I9e730a2d90a6174634c3233230e48e63549e1b9c
(cherry picked from commit af8e4965cb3702264c7e6f47fd9ba8b206a350da)
Merge "Fixing Neutron Subnet Details help text" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/horizon into stable/icehouse