increase the default allsky/tmp size during upgrades

[NiteRide]

follow on from AllskyTeam/allsky-modules#277

Problem

Creating a keolapse in ${ALLSKY_TMP} significantly decreases the time to create the file but can cause ${ALLSKY_TMP} to run out of space.
Using these settings created a 150 MB file:

• quality: high
• framerate 30fps
• resolution: 1080p

Fix

During an Allsky upgrade, if the size of ${ALLSKY_TMP} is less than the current standard size, then increase it and log the action to the terminal and to the install log.
The size of ${ALLSKY_TMP} depends on the amount of memory on the Pi because using "too much" memory for ${ALLSKY_TMP} on a Pi with 1/2 GB or 1 GB can cause a lot more swapping.

Current sizes are:

Pi memory   ${ALLSKY_TMP} MB
-----------------------------------
0.5 GB            75 MB
1 GB              150 MB
2 GB              200 MB
4 GB              300 MB
8 GB or more      400 MB

Should the 1 GB Pi change to 200 MB and change 2 GB Pi to 250 MB?

We'll need to either document that keolapse can't be created on Pi's with only 0.5 GB memory or change the keolapse code to create the file on the SD card.