[dependency loop] openstack-dashboard and horizon-vendor-theme packages

[Ivan Udovichenko]

Currently we have a loop dependency between openstack-dashboard [1] and horizon-vendor-theme [2] packages.
Although APT could install it anyway, better to get rid of it once and for all.
Dependency for horizon-vendor-theme in openstack-dashboard package is still required because we can't modify upstream Puppet manifests for that particular purpose. So we decided to remove openstack-dashboard dependency in horizon-vendor-theme package instead.

[1] https://review.fuel-infra.org/gitweb?p=openstack-build/horizon-build.git;a=blob;f=trusty/debian/control;h=34f8ac73ffa61f07b168589c5231fa84b965b359;hb=3e873db0ab9dff8a8d0fc40a1e8f07b13667c072#l162
[2] https://review.fuel-infra.org/gitweb?p=openstack-build/horizon-vendor-theme-build.git;a=blob;f=trusty/debian/control;h=3575c3a44919559a7383cbef87cb39cc91241eca;hb=e2be896f4d3de56c3f10316533b7ef70d2c3e385#l11

[Ivan Udovichenko (iudovichenko)]

Proposed fix:
https://review.fuel-infra.org/#/c/25925/