Add openSUSE Leap 15.2 tests

[Conan-Kudo]

Since DNF 4.x landed in openSUSE Leap 15.1 (which is in Beta currently), libmodulemd is now present in this distribution.

We should make sure we don't accidentally break building and using the library on this distribution (and prevent more issues like #177 or #180 from happening again).

[sgallagher]

This would be a good task for an Outreachy candidate. To get started, take a look at the .travis.yml and .travis/ directory in the source tree.

A new .travis/opensuse/ subdirectory should be created and populated with an appropriate Dockerfile and travis-tasks.sh.

[NishkaDG]

I would like to work on this.

[hafsasam]

Hello ! I am an Outreachy applicant. Can I work on this issue ?

[NishkaDG]

@hafsasam I am already working on this :)

[sgallagher]

This is open for anyone to work on.

[Conan-Kudo]

Current libmodulemd 2.x requires meson 0.47, which will land in openSUSE Leap 15.2, so retargeting this issue for that.