Add requests as a dependency, split install target

* Add requests as a dependency on all OSs

* Split the install target into a common target and separate
  install-systemd and install-openrc targets to support installing on
  Alpine

GENTOO/pgmon-1.0.3.ebuild

@@ -22,6 +22,7 @@ DEPEND="
 	${PYTHON_DEPS}
 	dev-python/psycopg:2
 	dev-python/pyyaml
+	dev-python/requests
 	app-admin/logrotate
 	"
 RDEPEND="${DEPEND}"