is_leader requires juju 1.22.0 or better

The ETCD charm fails with a non committal error message when probing for
is_leader on juju versions <= 1.21, 1.18 is still the default shipping
in universe, and will cause some headaches if users of older juju
versions attempt to use this charm as its leveraging the leader election
components.

There is probably a charm helper for consuming these methods that has a
sane fallback strategy, but for now this will satisfy the requirement of
blocking installation on anything < 1.22.0

Introduces a new dependency 'semver' to perform the version calculation

hooks/hooks.py

@@ -8,6 +8,7 @@
 from path import path
 import string
 import random
+import semver
 import shlex
 from subprocess import check_output, check_call
 import sys
@@ -24,6 +25,10 @@
 
 @hooks.hook('config-changed')
 def config_changed():
+    if not version_check:
+        hookenv.log('CRITICAL', 'This charm requires juju 1.22.0 or greater.'
+                                ' Panic and exit!')
+        sys.exit(1)
     if not db.get('installed') or hookenv.config().changed('source-sum'):
         install_etcd()
     if leader_status:
@@ -152,6 +157,10 @@ def install_etcd():
     hookenv.open_port(4001)
     db.set('installed', True)
 
+def version_check():
+    version = check_output(["juju", "--version"])
+    return semver.match(version, ">=1.22.0")
+
 if __name__ == '__main__':
     with hook_data():
         hooks.execute(sys.argv)

hooks/install

@@ -5,7 +5,7 @@ set -ex
 # corrected and easy_install is no longer required to make pip function.
 # apt-get install -y python-pip
 easy_install -U pip
-pip install charmhelpers path.py requests
+pip install charmhelpers path.py requests semver
 
 echo "Creating etcd data path on $JUJU_UNIT_NAME"
 mkdir -p /opt/etcd/var