make sure root partition is big enough

[Batcastle]

On systems where the root drive is small, and EFI was in use, and a
seperate /home partition was being used on a different drive, root
partitions where not being made big enough. Fixed that.

[pep8speaks]

Hello @Batcastle! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file usr/share/system-installer/auto_partitioner.py:

Line 110:22: E225 missing whitespace around operator
Line 127:80: E501 line too long (80 > 79 characters)
Line 199:80: E501 line too long (87 > 79 characters)
Line 202:80: E501 line too long (85 > 79 characters)
Line 204:80: E501 line too long (95 > 79 characters)
Line 206:80: E501 line too long (85 > 79 characters)
Line 209:80: E501 line too long (83 > 79 characters)
Line 221:80: E501 line too long (80 > 79 characters)
Line 223:80: E501 line too long (87 > 79 characters)
Line 260:80: E501 line too long (95 > 79 characters)
Line 262:80: E501 line too long (83 > 79 characters)
Line 265:80: E501 line too long (81 > 79 characters)
Line 267:80: E501 line too long (94 > 79 characters)
Line 269:80: E501 line too long (83 > 79 characters)
Line 271:80: E501 line too long (81 > 79 characters)
Line 284:80: E501 line too long (81 > 79 characters)
Line 287:80: E501 line too long (82 > 79 characters)
Line 295:80: E501 line too long (85 > 79 characters)
Line 303:80: E501 line too long (82 > 79 characters)
Line 305:80: E501 line too long (135 > 79 characters)
Line 306:80: E501 line too long (81 > 79 characters)
Line 307:80: E501 line too long (95 > 79 characters)
Line 308:80: E501 line too long (113 > 79 characters)
Line 310:80: E501 line too long (93 > 79 characters)
Line 312:80: E501 line too long (133 > 79 characters)
Line 313:80: E501 line too long (81 > 79 characters)
Line 314:80: E501 line too long (95 > 79 characters)
Line 315:80: E501 line too long (117 > 79 characters)
Line 316:80: E501 line too long (93 > 79 characters)
Line 317:80: E501 line too long (141 > 79 characters)
Line 320:80: E501 line too long (141 > 79 characters)
Line 333:17: E722 do not use bare 'except'
Line 338:80: E501 line too long (100 > 79 characters)
Line 413:80: E501 line too long (96 > 79 characters)
Line 482:80: E501 line too long (86 > 79 characters)
Line 493:80: E501 line too long (80 > 79 characters)
Line 564:80: E501 line too long (85 > 79 characters)
Line 599:80: E501 line too long (92 > 79 characters)
Line 600:80: E501 line too long (100 > 79 characters)
Line 614:80: E501 line too long (80 > 79 characters)