Compatibility with GnuPG 1.x and 2.x, auto-detect GnuPG version

* aptly can sign and verify without issues with GnuPG 1.x and 2.x
* aptly auto-detects GnuPG version and adapts accordingly
* aptly automatically finds suitable GnuPG version

Majority of the work was to get unit-tests which can work with GnuPG 1.x & 2.x.
Locally I've verified that aptly supports GnuPG 1.4.x & 2.2.x. Travis CI
environment is based on trusty, so it runs gpg2 tests with GnuPG 2.0.x.

Configuration parameter gpgProvider now supports three values for GnuPG:

* gpg (same as before, default): use GnuPG 1.x if available (checks gpg, gpg1),
otherwise uses GnuPG 2.x; for aptly users who already have GnuPG 1.x
environment (as it was the only supported version) nothing should change; new
users might start with GnuPG 2.x if that's their installed version

* gpg1 looks for GnuPG 1.x only, fails otherwise

* gpg2 looks for GnuPG 2.x only, fails otherwise

system/lib.py

@@ -142,7 +142,16 @@ class BaseTest(object):
                                                      self.fixtureWebServer))
 
         if self.fixtureGpg:
-            self.run_cmd(["gpg", "--no-default-keyring", "--trust-model", "always", "--batch", "--keyring", "aptlytest.gpg", "--import"] +
+            # try to find gpg1 as that's what aptly prefers by default to build trusted keys in DB
+            # in lowest supported format
+            gpg = "gpg1"
+            try:
+                subprocess.check_output(["gpg1", "--version"])
+            except Exception:
+                gpg = "gpg"
+
+            # TODO: fixme
+            self.run_cmd([gpg, "--no-default-keyring", "--trust-model", "always", "--batch", "--keyring", "aptlytest.gpg", "--import"] +
                          [os.path.join(os.path.dirname(inspect.getsourcefile(BaseTest)), "files", key) for key in self.fixtureGpgKeys])
 
         if hasattr(self, "fixtureCmds"):