travis-ci: make dnf invocations more resilient
Travis-CI sometimes fails to download repository metadata or
packages. Change dnf configuration and invocation:
* activate dnf fastestmirror
* add more dnf retries
* invoke "dnf makecache" twice
Fixes: https://pagure.io/freeipa/issue/8048
Signed-off-by: François Cami <fcami@redhat.com>
Reviewed-By: Scott Poore <spoore@redhat.com>