Define scan configuration parameter for variable time between executions

YAML configuration:

```
sensor:
  - platform: ssh
      host: !secret proxmox_host
      name: 'NUC CPU Temp'
      username: !secret proxmox_user
      password: !secret proxmox_pass
      command: "sensors | grep 'Package id 0:' | cut -c17-20"
      value_template: >-
        {%- set line = value.split("\r\n") -%}
        {{ line[1] }}
      unit_of_measurement: "ºC"
     # Time between executions, in seconds
      scan: '900'
```

custom_components/ssh/sensor.py

@@ -39,6 +39,7 @@
     vol.Required(CONF_COMMAND): cv.string,
     vol.Required(CONF_UNIT_OF_MEASUREMENT): cv.string,
     vol.Optional(CONF_VALUE_TEMPLATE): cv.template,
+    vol.Optional(CONF_SCAN, default=MIN_TIME_BETWEEN_UPDATES): cv.string,
 })
 
 @asyncio.coroutine
@@ -61,6 +62,7 @@ def __init__(self, hass, config):
         self._command = config.get(CONF_COMMAND)
         self._value_template = config.get(CONF_VALUE_TEMPLATE)
         self._unit_of_measurement = config.get(CONF_UNIT_OF_MEASUREMENT)
+        self._scan = config.get(CONF_SCAN)
         self._ssh = None
         self._connected = False
         self._connect()
@@ -94,7 +96,12 @@ def unit_of_measurement(self):
         """Return the unit of measurement of this entity, if any."""
         return self._unit_of_measurement
 
-    @Throttle(MIN_TIME_BETWEEN_UPDATES)
+    @property
+    def scan(self):
+        """Return the period between executions, if any."""
+        return self._scan
+
+    @Throttle(self._scan)
     def update(self):
         from pexpect import pxssh, exceptions
 
@@ -128,7 +135,7 @@ def update(self):
             return None
 
     def _connect(self):
-        """Connect to the Unifi AP SSH server."""
+        """Connect to the SSH server."""
         from pexpect import pxssh, exceptions
 
         self._ssh = pxssh.pxssh()