Conditional creation of install_requires of rpm distribution (#114)

This commit adds conditinal creation of install_requires list
in setup.py for rpm distribution.
If rpm-py-installer was added there unconditionally,
rpm of ubipop couldn't be installed on some envs.
Implemented in this way there no need to change
anything in .spec file for rpm builds.

requirements.txt

@@ -1,4 +1,3 @@
 requests
 more-executors
-ubi-config
-rpm-py-installer
+ubi-config

setup.py

@@ -1,4 +1,5 @@
 from setuptools import setup
+import pkg_resources
 
 
 def get_description():
@@ -14,9 +15,24 @@ def get_long_description():
     return text[idx:]
 
 
+def get_rpm_distribution():
+    for distribution in ['rpm', 'rpm-python']:
+        try:
+            pkg_resources.get_distribution(distribution)
+        except pkg_resources.DistributionNotFound:
+            continue
+        else:
+            return distribution
+    return 'rpm-py-installer'
+
+
 def get_requirements():
+    requirements = [get_rpm_distribution()]
+
     with open('requirements.txt') as f:
-        return f.read().splitlines()
+        requirements.extend(f.read().splitlines())
+
+    return requirements
 
 
 setup(name='ubi-population-tool',